Methods for multiplexing recombinase polymerase amplification

ABSTRACT

This disclosure provides for methods and reagents for rapid multiplex RPA reactions and improved methods for detection of multiplex RPA reaction products. In addition, the disclosure provides new methods for eliminating carryover contamination between RPA processes.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/066,334 filed Oct. 29, 2013, which is a continuation of U.S.application Ser. No. 13/177,007 filed Jul. 6, 2011, which is acontinuation of U.S. application Ser. No. 11/988,825 filed Jun. 2, 2009,which is a National Stage Application of PCT/IB06/04113 filed Jul. 25,2006, and claims the benefit of priority from U.S. Application No.60/702,533 filed Jul. 25, 2005 and U.S. Application No. 60/728,424 filedOct. 18, 2005. The contents of the prior applications are incorporatedby reference herein.

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ELECTRONICALLY

Incorporated by reference in its entirety herein is a computer-readablenucleotide/amino acid sequence listing submitted concurrently herewithand identified as follows: One 24 kilobyte ASCII (Text) file named“35622-408_SEQUENCE_LISTING_ST25.txt.” created on Jun. 2, 2021.

BACKGROUND

Recombinase Polymerase Amplification (RPA) is a DNA amplificationprocess that utilizes enzymes to match synthetic oligonucleotide primersto their complementary partners in duplex DNA. (Armes and Stemple, U.S.patent Appl. 60/358,563 filed Feburuary 21, 2002). RPA depends uponcomponents of the cellular DNA replication and repair machinery. Thenotion of employing some of this machinery for in vitro DNAamplification has existed for some time (Zarling et al. U.S. Pat. No.5,223,414), however the concept has not transformed to a workingtechnology until recently as, despite a long history of research in thearea of recombinase function involving principally the E. coli recAprotein, in vitro conditions permitting sensitive amplification of DNAhave only recently been determined (Piepenburg et al. U.S. patentapplication Ser. No. 10/931,916 filed Sep. 1, 2004, also Piepenburg etal., PlosBiology 2006).

RPA offers a number of advantages over traditional methods of DNAamplification. These advantages include the lack of a need for anyinitial thermal or chemical melting, the ability to operate at lowconstant temperatures without a need for absolute temperature control,as well as the observation that complete reactions (lacking target) canbe stored in a dried condition. These characteristics demonstrate thatRPA is a uniquely powerful tool for developing portable, accurate, andinstrument-free nucleic acid detection tests.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to methods of nucleic acid amplificationwhich include novel recombinase polymerase amplification (RPA) protocolsfor rapid and efficient amplification of nucleic acids in a process thatcan be easily multiplexed.

One embodiment of the invention is directed to a method wherein aplurality of RPA which can be performed simultaneously in a singlereaction (in a single tube) and wherein the results may be detectedsimultaneously. The single RPA reaction is described first below andmethods of multiplexing said reaction is described second.

One aspect of the invention is directed to methods of RPA whichgenerates easily detectable amplimers (an amplified nucleic acid whichis the product of an RPA reaction). The RPA process amplified a doublestranded target nucleic acid molecule comprising a first and a secondstrand of DNA. Step (a) involves contacting a recombinase agent with afirst and a second nucleic acid primer and a third extension blockedprimer which comprises one or more noncomplementary or modified internalresidue to form a first, second and third nucleoprotein primer. Step (b)involves contacting the first and second nucleoprotein primers to saiddouble stranded target nucleic acid thereby forming a first doublestranded structure between said first nucleoprotein primer and saidfirst strand of DNA at a first portion of said first strand (forming a Dloop) and a second double stranded structure between said secondnucleoprotein primer and said second strand of DNA at a second portionof said second strand (forming a D loop) such that the 3′ ends of saidfirst nucleoprotein primer and said first nucleoprotein primer areoriented toward each other on the same target nucleic acid molecule witha third portion of target nucleic acid between said 3′ ends; Step (c)involves extending the 3′ end of said first nucleoprotein primer andsecond nucleoprotein primer with one or more polymerases and dNTPs togenerate a first amplified target nucleic acid with an internal regioncomprising the third portion of nucleic acid. Step (d) involvescontacting said amplified target nucleic acid to said thirdnucleoprotein primer to form a third double stranded structure at thethird portion of said amplified target nucleic acid (forming a D loop)in the presences of a nuclease; wherein said nuclease specificallycleaves said noncomplementary internal residue only after the formationof said third double stranded structure to form a third 5′ primer and athird 3′ extension blocked primer. Step (d) involves extending the 3′end of said third 5′ primer with one or more polymerase and dNTP togenerate a second double stranded amplified nucleic acid which comprisessaid first nucleic acid primer and said third 5′ primer. The RPAreaction is continued until a desired degree of the second doublestranded amplified nucleic acid is reached. It should be noted that thisprocess, along with any related embodiments, may be used for multiplexRPA reaction (described below).

The recombinase agent may be, for example, uvsX, RecA and functionalanalogs thereof. Further, the RPA reaction may be performed in thepresence of uvxY, gp32, single strand binding proteins and other usualRPA reagents. Methods for performing RPA are disclosed, for example, inU.S. Appl. 60/358,563 filed Feb. 21, 2002, U.S. application Ser. No.10/371,641, filed Feb. 21, 2003, 2003, U.S. patent application Ser. No.10/931,916 filed Sep. 1, 2004 and PCT/IB2005/001560 (WO2005/118853)filed Apr. 11, 2005.

The nuclease used in this RPA reaction should specifically cleave thenoncomplementary residue or the modified internal residue preferentiallywhen the third extension blocked primer is hybridized to a DNA to form adouble stranded structure. It is preferred that the nuclease do notcleave the noncomplementary residue or the modified internal residuewhen the extension blocked primer is in single stranded form—regardlessof whether the primer is attached to recombinase or SSB. In a preferredembodiment, the nuclease is a DNA glycosylase or AP endonuclease. If themodified internal residue is a uracil or inosine, the preferred nucleaseis uracil glycosylase or hypoxanthine-DNA glycosylase respectively. Thenuclease may recognize the noncomplementary base by nature of a mismatchwhich forms a region of noncomplementary residues (i.e., a bubble) in anotherwise double stranded structure. In this case, the nucleaserecognizes a base mismatch between the noncomplementary residues andcleaves primer at the noncomplementary base.

The nuclease used in any of the processes of the invention may be a DNAglycosylase or an AP endonuclease. The nuclease may function byrecognizing a base mismatch between said first extension blocked primerand said target nucleic acid and cleaving the extension blocked primerat the base mismatch without cleaving the target nucleic acid. Thenuclease, alternatively, may recognize a damaged residue, an abasic siteor abasic site mimic, or any other modification which may beincorporated into synthetic oligonucleotides. The nuclease may be, forexample, fpg, Nth, MutY, MutS, MutM, E. coli. MUG, human MUG, humanOgg1, a vertebrate Nei-like (Neil) glycosylases, Nfo, exonuclease III,uracil glycosylase, hypoxanthine-DNA and functional analogs and homologsthereof. The functional analogs and homologs may be of any mammalian,bacterial or viral original. As additional examples, if the modifiedbase is inosine, the nuclease may be hypoxanthine-DNA glycosylase; ifthe modified base is uracil, the nuclease may be uracil glycosylase. Ina preferred embodiment, these nucleases may be from E. coli. In apreferred embodiment, the nuclease is E. coli Nfo or E. coli exonucleaseIII and the modified internal residue is a tetrahydrofuran residue or alinker group. A ‘linker’ (also called a carbon linker or ‘spacer’) is acarbon-containing chain which is used to join the 3′ position of onesugar to the (usually) 5′ position of another. Common spacers maycomprise about 3, 6, 9, 12 or 18 carbon chains although it may be of anynumber of carbon chains. Carbon-oxygen-carbon linkages are common inthese spacers, presumably to reduce hydrophobicity. Nfo and exonucleaseIII (and homologs) can recognize the sugar 3′-O—C linkage on the 3′ endof a nucleotide linked to a spacer and cleave it. See, for example, C18spacer (18-O-Dimethoxytritylhexaethyleneglycol, 1-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite (Glen Research, Sterling, Va., USA, cat#10-1918-90).

As used herein, an “abasic residue” in an oligonucleotide refers to amolecular fragment (MF) within an oligonucleotide chain where themolecular fragment approximates the length of a ribofuranose or adeoxyribofuranose sugar in such a way that bases adjacent to themolecular fragment are separated from one another by the same, oreffectively the same, distance as if a ribofuranose or adeoxyribofuranose sugar of any of A, G, C, T, or U were present in placeof the abasic residue. The abasic residue may incorporate a ribofuranoseor deoxyribofuranose ring as in native A, G, C, T, or U. However, theabasic residue does not contain a base or other molecule that caninteract with the base on the opposite strand of a duplex which isformed with the abasic residue-containing oligonucleotide. Thus, anabasic residue may be an apurine or apyrimidine structure, a baseanalog, or an analogue of a phosphate backbone. The abasic substitutionmay also consist of a backbone of N-(2-aminoethyl)-glycine linked byamide bonds. In a preferred embodiment, the abasic residue istetrahydrofuran or D-spacer (a type of tetrahydrofuran). Both a D-spacerand tetrahydrofuran effectively are a deoxyribose sugar in which boththe 1′ and 2′ position lack OH residues. Normally the 1′ position of atrue abasic residue in DNA would have a hydroxyl in the position wherethe base is normally attached, however this is unstable as the ring forminterconverts with an open-ring aldehyde form (see below) which can thendegrade by the process of beta-elimination. Removal of this hydroxylleads to a stable form readily synthesized into oligonucleotides.Tetrahydrofuran-type abasic sites and their use as abasic residues areknown. The tetrahydrofuran may be placed into oligonucleotides duringsynthesis by ordering reagents from Glen Research (Sterling, Va., USA).

The one or more noncomplementary or modified internal residue isinternal because it is not the 5′ most or 3′ most residue of the firstextension blocked primer. In a preferred embodiment, the one or morenoncomplementary internal residue is at least 10 residues away from the5′ or 3′ residue of a primer. In a more preferred embodiment, the one ormore noncomplementary internal residue is at least 15, or at least 20residues away from the 5′ or 3′ residue of a primer.

The one or more noncomplementary internal residue may be introduced bysynthesizing an oligonucleotide primer with one or more noncomplementaryresidue. A noncomplementary residue is any residue that does not form aWatson Crick base pair (hydrogen bond) with its corresponding residue ina double stranded structure. For example, if a “T” at a particularlocation is needed to form a Watson-Crick base pair between a primer anda target nucleic acid, the use of an “A” would cause the “A” to be noncomplementary. As a further example, each of the middle bases in thefollowing double stranded structure is a noncomplementary base.

primer aaaaa (SEQ ID NO: 1) || || target ttatt (SEQ ID NO: 2) primeraagaa (SEQ ID NO: 3) || || target ttatt (SEQ ID NO: 4) primeraacaa (SEQ ID NO: 5) || || target ttatt (SEQ ID NO: 6)

It is known that the presence of noncomplementary residues in a doublestranded nucleic acid will produce a bubble within the double strandednucleic acid. While one noncomplementary or modified internal residue issufficient for functioning with the methods of the invention, more thanone noncomplementary or modified internal residues may be used. Whenmore than one is used, they may adjacent to each other on anoligonucleotide or they may be separated. It should be noted that if thenuclease cleaves the target nucleic acid at the mismatch ornoncomplementary location, the target DNA is repaired rapidly by dNTPand polymerase using the primer as a template. Because of this, thisreaction would not affect the processes of this disclosure.

The one or more noncomplementary internal residue of the first extensionblocked primer may be a modified internal residue. The modified internalresidue may be any chemical structure (residue) that cannot form aWatson-Crick base pairing structure with its corresponding base in adouble stranded nucleic acid structure. If more than onenoncomplementary internal residue is used, they can be a mixture ofnoncomplementary internal residues or modified internal residues. Theterm “modified internal residue,” also includes, at least, any residuenot normally found in DNA—that is any residue which is not an “A”, “G”,“C” or “T” such as, for example uracil or inosine.

The modified internal residue may be inosine, uracil, 8-oxoguanine,thymine glycol, or an abasic site mimic. Preferred abasic site mimicsinclude a tetrahydrofuran residue or D-spacer (which can be produced asa product of employing a5′-O-Dimethoxytrityl-1′,2′-Dideoxyribose-3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramiditeduring oligonucleotide synthesis.

The extension blocked primer is blocked at its 3′ end so that it cannotnormally be elongated by polymerase and dNTP even in the presence of acomplimentary template. Methods of blocking a primer are well known andinclude, at least, the inclusion of a blocked 3′ nucleotide. The blocked3′ nucleotide may contain, for example, a blocking group that preventspolymerase extension. Generally, the blocking groups are attached to the3′ or 2′ site of the 3′ sugar residue but other locations of attachmentsare possible. One of the most common 3′ blocking methods is to place adideoxy sugar at the 3′ end of an oligonucleotide. The blocking groupmay be, for example, a detectable label.

A detectable label is defined as any moiety that may be detected usingcurrent methods. These labels include, at least, a fluorophore (alsocalled a fluorescent molecule, fluorochrome), an enzyme, a quencher, anenzyme inhibitor, a radioactive label, a member of a binding pair, adigoxygenin residue, a peptide, and a combination thereof.

“A member of a binding pair” is meant to be one of a first and a secondmoiety, wherein said first and said second moiety have a specificbinding affinity for each other. Suitable binding pairs for use in theinvention include, but are not limited to, antigens/antibodies (forexample, digoxigenin/anti-digoxigenin, dinitrophenyl (DNP)/anti-DNP,dansyl-X-anti-dansyl, Fluorescein/anti-fluorescein, luciferyellow/anti-lucifer yellow, peptide/anti-peptide, ligand/receptor andrhodamine/anti-rhodamine), biotin/avidin (or biotin/streptavidin) andcalmodulin binding protein (CBP)/calmodulin. Other suitable bindingpairs include polypeptides such as the FLAG-peptide (DYKDDDDK; SEQ IDNO:7) [Hopp et al., BioTechnology, 6:1204 1210 (1988)]; the KT3 epitopepeptide (Martin et al., Science 255:192 194 (1992)); tubulin epitopepeptide (Skinner et al., J. Biol. Chem 266:15163 15166 (1991)); and theT7 gene 10 protein peptide tag (Lutz-Freyermuth et al., Proc. Natl.Acad. Sci. USA, 87:6393 6397 (1990)) and the antibodies each thereto.Generally, in a preferred embodiment, the smaller of the binding pairpartners serves as the detectable label, as steric considerations may beimportant. In addition to the above, any of the nucleic acid andnucleotides of the RPA reaction may be labeled with a detectable label.

In any of the RPA processes of the invention where a detectable label isused, the detectable label may be used to monitor the progress (theproduction of amplimers) of the RPA reaction. In one aspect, if theprimers are labeled, monitoring may involve detecting a label in anamplimer. Since amplimers would be expected to be larger than theprimers used, detection may involve, for example gel electrophoresis andthe detection of the proper sized amplimer. Alternatively, labeledamplimers may be separated by labeled primers by a more rapid processsuch as column chromatography (including spin columns, push columns andthe like). Since the RPA methods of the invention has high specificityand low artifact production (high signal to noise), monitoring mayinvolve performing RPA using nucleotides attached to detectable labelsand measuring the amount of labels attached to high molecular weightnucleic acid (e.g., nucleic acid of more than 100 bases in length). Forexample, radioactive dNTPs may be used and the progress of the RPAreaction may be monitored by following the incorporation of radiationinto high molecular weight DNA. Techniques that monitor incorporation ofnucleotides into high molecular weight DNA include gel electrophoresis,size exclusion column (e.g., conventional, spin and push columns) andacid precipitation.

If the first nucleic acid primer and the third 5′ primer are eachlabeled with a different detectable label, then the amplified product(the second double stranded amplified nucleic acid) will be the onlynucleic acid species with both labels. This double labeled nucleic acidspecies may be detected by a variety of means. In one preferred method,the amplified product may be detected using a flow strip. In onepreferred embodiment, one detectable label produces a color and thesecond label is an epitope which is recognized by an immobilizedantibody. A product containing both labels will attach to an immobilizedantibody and produce a color at the location of the immobilizedantibody. An assay based on this detection method may be, for example, aflow strip (dip stick) which can be applied to the whole RPA reaction. Apositive amplification will produce a band on the flow strip while anegative amplification would not produce any color band.

It should be noted that this RPA amplification process using 3 primersmay be multiplexed (referred to herein as multiplex RPA). That is,multiple RPA process using 3 primers, as discussed above, may beperformed in the same reaction (tube). Multiplex RPA may be performedwith one or more target nucleic acids. Each process is performed with adifferent combination of first and second nucleic acid primers which isspecific for a different region of one or more target nucleic acids. Ina preferred embodiment, when multiple RPA processes are performed in thesame reaction, each RPA process uses a first nucleic acid with the samelabel but not necessarily the same sequence. Further, each process usesthe same third extension blocked primer with a second detectable label.In this way, by measuring the accumulation of double stranded nucleicacid product with both the first detectable label and the seconddetectable label, the cumulative amplification of each RPA process maybe measured.

Multiplexed RPA is useful for many purposes. For example, multiplepathogens may share a common nucleic acid sequence that is too small fordirect amplification by RPA. Furthermore, the common nucleic acidsequence have different flanking sequence in each organism so that asingle set of RPA primers cannot be designed to amplify this commonnucleic acid sequence in multiple organisms. Using the process ofmultiplex RPA as described above, a plurality of combination of RPAprimers may be used in one reaction, wherein each combination wouldamplify the common nucleic acid sequence in one organism and this commonnucleic acid sequence would be concomitantly amplified by the commonthird primer (third extension blocked primer). Multiplex RPA with primercombinations designed to detect multiple pathogens, may be used forexample, in an assay to detect methicillin resistant S. aureus strainsby amplifying and detecting a common sequence (e.g., mec2) in eachstrain. By using the multiplexed RPA of the invention, a plurality ofloci (DNA sequences) may be detected by concurrent RPA amplification. Ina preferred embodiment, at least 2 simultaneous RPA are performed in anRPA. In a more preferred embodiment, at least 3, at least 5, at least 7or at least 10 RPA reactions may be performed in the same tube.

Thus, another aspect of the invention is directed to a multiplex methodof RPA comprising the steps of performing more than one RPA process inone reaction. Each individual reaction is performed as described abovefor RPA using 3 primers. Briefly, each reaction involves the steps of(a1) contacting a recombinase agent with a first and a second nucleicacid primer and a third extension blocked primer which comprises anoncomplementary or modified internal residue to form a first, secondand third nucleoprotein primer; (a2) contacting the first and secondnucleoprotein primers to said double stranded target nucleic acidthereby forming a first double stranded structure between said firstnucleoprotein primer and said first strand of DNA at a first portion ofsaid first strand and a second double stranded structure between saidsecond nucleoprotein primer and said second strand of DNA at a secondportion of said second strand such that the 3′ ends of said firstnucleoprotein primer and said first nucleoprotein primer are orientedtoward each other on the same target nucleic acid molecule with a thirdportion of nucleic acid between said 3′ ends; (a3) extending the 3′ endof said first nucleoprotein primer and second nucleoprotein primer withone or more polymerases and dNTPs to generate a first amplified targetnucleic acid with an internal region comprising the third portion ofnucleic acid; (a4) contacting said amplified target nucleic acid to saidthird nucleoprotein primer to form a third double stranded structure atthe third portion of said amplified target nucleic acid in the presencesof a nuclease; wherein said nuclease specifically cleaves saidnoncomplementary or modified internal residue only after the formationof said third double stranded structure to form a third 5′ primer and athird 3′ extension blocked primer; (a5) extending the 3′ end of saidthird 5′ primer to generate a second double stranded amplified nucleicacid which comprises said first nucleic acid primer and said third 5′primer; (a6) continuing the reaction through repetition of (a2) and (a5)until a desired degree of the second double stranded amplified nucleicacid is reached. In this process, each RPA process is performed with adifferent combination of first and second nucleic acid primers but eachprocess is performed with the same third extension blocked primer.

It should be noted that while each RPA process will have a differentcombination of first and second nucleic acid primers, primers can stillbe shared between RPA processes. For example, RPA process 1 may useprimers 1 and 2 while RPA process 2 may use primers 2 and 3. Thus, RPAprocess 1 and RPA process 2 share the same primer (primer 2).

In any RPA process that involves an extension blocked primer (e.g., thethird extension blocked primer) the primer may further comprises one ormore detectable labels and the progress of the RPA may be monitored asecond way by monitoring the detectable label on this primer. Thedetectable label may be a fluorophore, an enzyme, a quencher, an enzymeinhibitor, a radioactive label, one member of a binding pair and acombination of thereof. Where a fluorophore or quencher is used, theattachment may be by a fluorophore-dT amidite residue or a quencher-dTamidite residue.

In a preferred embodiment, the third extension blocked primer comprisesa fluorophore and a quencher. The fluorophore and quencher are separatedby between 0 to 2 bases, 0 to 5 bases, 0 to 8 bases or 0 to 10 bases, 3to 5 bases, 6 to 8 bases, or 8 to 10 bases. In addition, the fluorophoreand quencher may be separated by a greater distance when the extensionblocked primer is unhybridized than when the extension blocked primer ishybridized to the target nucleic acid. Furthermore, the fluorophore orquencher may be attached to the noncomplementary or modified internalresidue as long as the fluorophore and quencher are separated followingcleavage of the modified internal base by the nuclease. Preferredfluorophores include fluorescein, FAM, TAMRA and preferred quenchersinclude a dark quencher (e.g., Dark Quencher 1, Dark Quencher 2, BlackHole Quencher 1 and Black Hole Quencher 2).

One advantage of the methods of this RPA process is that it can beperformed at a low temperature such as between 14° C. and 21° C.,between 21° C. and 25° C., between 25° C. and 30° C., between 30° C. and37° C. or between, 40° C. and 43° C. Under these temperature conditions,the reaction are accelerated in the presence of 1% to 12% PEG such asbetween 6% to 8% PEG.

Another advantage of using extension blocked primers, for any of themethods of the invention, is that the progress of the reaction may bemonitored in real time. Monitoring may involve, for example, measuringfluorescence in the RPA reaction. In this method, the fluorophore andquencher are located at a sufficiently close distance (less than 10residues apart, as disclosed in this specification) on the primer suchthat the quencher prevents fluorescence from the fluorophore. However,as the extension blocked primer is cleaved by the nuclease, the quencheris separated from the fluorophore and the primer becomes fluorescent.This allows the monitoring of RPA in real time, merely by using a lightsource which can excite the fluorophore to fluoresce and using anoptical detector to detect any fluorescence from the fluorophore whichhas separated from the quencher.

The primers for any of the RPA reactions of this disclosure, includingthe extension blocked primers, may be between 2 to 100 residues inlength, such as between 12 to 30 residues in length, 12 to 40 residuesin length, 12 to 50 residues in length, or 12 to 60 residues, 30 to 40residues in length, 40 to 45 residues in length, or 45 to 50 residues inlength. In a preferred embodiment, the primers may be between 30 to 100,between 35 to 100, between 40 to 100 or between 45 to 100 in length. Inthe most preferred embodiment, the primers are between 30 to 60 inlength, between 35 to 60, between 40 to 60 or between 45 to 60 inlength—these primers may be used in any RPA reactions and are especiallypreferred for RPA reactions below 30° C. degrees, below 15° C. degreesor below 20° C. Primers lengths of greater than 30, greater than 35,greater than 40, greater than 45 or greater than 50 bases are preferredfor RPA processes performed at or below 30° C. It is understood that inthe field of molecular biology, the subunits of a nucleic acid arereferred to as “bases” or “residues.” For example, DNA andoligonucleotide structures and lengths are referred to in bases(kilobases), basepairs or residues.

Any of the RPA reaction of the invention may be performed between 14° C.and 21° C., between 21° C. and 25° C., between 25° C. and 30° C.,between 30° C. and 37° C., between 38° C. to 40° C. or between 40° C.and 48° C. Applicants have found that RPA reactions are optimal at 25°C. in the presence of between 1% to 12% percent PEG. Preferably, theconcentration of PEG is between 6 to 9% such as, for example between 7to 8%. These optimal RPA conditions applies to the RPA reactionsdisclosed in this application and to all RPA reactions in general.

In a typical RPA reaction of the invention, at least one strand of thetarget nucleic acid is amplified at least 10′ folds, at least 10⁸ foldsor at least 10⁹ folds.

For any of the RPA methods of the invention, it is understood that thetarget nucleic acid may be single stranded. Single stranded nucleic acidmay be converted to double stranded nucleic acid by methods known in theart including, for example, the hybridization of random primers followedby elongation by polymerase. Furthermore, the RPA reaction may beperformed directly with single stranded target nucleic acid because in afirst step, a RPA primer would hybridize to the single stranded targetnucleic acid and extension (in the presence of nuclease in the case ofthe extension blocked primer) by polymerase and dNTPs would generate adouble stranded target nucleic acid for subsequent RPA. Further, aspecific primer may be added at the beginning of the RPA reaction tohybridize to the single stranded target nucleic acid and by extensionwith polymerase already present in the RPA reaction, convert the singlestranded target nucleic acid into a double stranded target nucleic acid.

To reduce background and contamination, any of the RPA reactions of theinvention may be performed with dUTP in the dNTP mix. We have found,surprisingly, that an RPA may be performed in the presence of dUTP andactive uracil glycosylase for a first period before the uracilglycosylase is inactivated. This first period is preferably less than 20minutes, less than 10 minutes, less than 5 minutes or less than 2minutes. Furthermore, the uracil glycosylase may be added at any timeduring the first period. That is, the RPA reaction may be started withdUTP (and other dNTPs) without uracil glycosylase and the uracilglycosylase may be added at any time during the first period.

After the first period, uracil glycosylase inhibitor is added to the RPAreaction and the reaction is allowed to continue for the remainder ofthe RPA reaction—until a desired degree of amplification is reached.Importantly, the process is performed without temperature basedinactivation of the uracil glycosylase. The uracil glycosylase inhibitorin this reaction may be a Bacillus subtilis phages PBS1 uracilglycosylase inhibitor or Bacillus subtilis phages PBS2 uracilglycosylase inhibitor. Where dUTP is used, for any RPA of thisdisclosure, the dNTP may consist of (1) dTTP, dATP, dUTP, dCTP and dGTPor (2) dATP, dUTP, dCTP and dGTP. In a preferred embodiment, when dUTPis used, the dNTP mixture does not contain dTTP. This method of reducingbackground, by adding dUTP and uracil glycosylase to a first portion ofan RPA reaction has general applicability to any type of RPA. Further,this method may be combined with any of the RPA processes of thisdisclosure.

Another aspect of the invention relates to a method of performing RPA ofa double stranded target nucleic acid molecule comprising a first and asecond strand of DNA with an increased signal to noise ratio. In step A,a recombinase agent is contacted with (1) a first extension blockedprimer which comprises one or more noncomplementary or modified internalresidue which can be a modified internal residue, and (2) a secondnucleic acid primer to form a first and a second nucleoprotein primer.

In step B, the first and second nucleoprotein primers are mixed with(contacted to) a nuclease and to the double stranded target nucleic acidsuch that a first double stranded structure (part of a first D-loop)between the first nucleoprotein primer and said first strand of DNA at afirst portion of said first strand is formed. Furthermore, a seconddouble stranded structure (part of a second D loop) between said secondnucleoprotein primer and said second strand of DNA at a second portionof said second strand is also formed. The 3′ ends of the first extensionblocked primer and said second nucleic acid primer are oriented towardeach other on the same double stranded target nucleic acid molecule. Thenuclease specifically recognizes and cleaves the one or morenoncomplementary or modified internal residue in the first extensionblocked primer only after the primer forms a double stranded structure.After cleavage by the nuclease, the first extension blocked primer iscleaved into two primers, a first 5′ primer and a first 3′ extensionblocked primer. Because the blocking group is on the 3′ end of the firstextension blocked primer, the first 5′ primer is not blocked but thefirst 3′ extension blocked primer is blocked and cannot be elongated bypolymerase.

In step C, the 3′ end of the first 5′ primer and second nucleoproteinprimer is extended with one or more polymerases and dNTPs (e.g., amixture of dATP, dTTP, dCTP, and dGTP) to generate an amplified targetnucleic acid. The amplified target nucleic acid may be single stranded(for example a displaced strand) or double stranded. Furthermore, singlestranded amplified target nucleic acid may hybridize to form doublestranded target nucleic acid. Furthermore, the RPA system of thisdisclosure can amplify both single stranded target nucleic acid(discussed below) or double stranded target nucleic acid so theproduction of single stranded or double stranded amplified targetnucleic acid would not affect the outcome of RPA.

Step B and step C are repeated until a desired degree of amplificationis reached. It should be noted that the RPA reaction is selfperpetuating as long as the reagents do not run out. The product of oneround of amplification (amplified target nucleic acid) serves as theinput for subsequent round of RPA. Thus, an RPA reaction may becontinued by merely continued incubation of the reaction at a desiredtemperature. Furthermore, since the RPA reaction disclosed is nottemperature sensitive, the reaction may be continued even if there iffluctuation in the temperature. For example, a RPA reaction tube may beperformed in a waterbath, on the bench top (room temperature), or evenin the pocket of the experimentor (when working in the field, forexample). Thus, the RPA reaction may be performed at less than 50° C.,less than 40° C., less than 37° C., less than 30° C., less than 25° C.,or less than 20° C.

In a preferred embodiment, the first extension blocked primer furthercomprises one or more detectable labels. Where the detectable label is afluorophore or a quencher, it may be attached to the extension blockedprimer by a fluorophore-dT amidite residue or quencher-dT amiditeresidue respectively. Other attachments are possible and widely known.

In another preferred embodiment, the extension blocked primer comprisesboth a fluorophore and a quencher. The fluorophore and quencher may beseparated by between 0 to 2 bases, 0 to 5 bases, 0 to 8 bases or 0 to 10bases. Naturally, it is preferred that the fluorophore and the quencherbe sufficiently close to each other such that the combination is notfluorescent until they are separated. It is preferred that thefluorophore and quencher are separated by a greater distance in thenucleoprotein primer than when the primer is hybridized to the targetnucleic acid. This is possible because of the action of the attachedproteins (recombinase and/or SSB protein) which tend to stretch out theunhybridized primer.

In another aspect, either fluorophore or the quencher may be attached tothe modified internal residue and the fluorophore and quencher can beseparated following cleavage of the modified internal residue by thenuclease.

While any fluorophore may function for the methods of the invention,fluorescein, FAM and TAMRA are preferred fluorophores. The preferredquencher is a dark quencher which may be, for example, Dark Quencher 1,Dark Quencher 2, Black Hole Quencher 1 or Black Hole Quencher 2.

Another aspect of the invention is directed to an RPA process of DNAamplification of a single stranded target nucleic acid moleculecomprising the steps of (a) hybridizing a first nucleic acid primer tosaid single stranded target nucleic acid and elongating said primer oneor more polymerases and dNTPs to generate a double stranded targetnucleic acid molecule comprising a first and a second strand; (b)contacting a recombinase agent with a first extension blocked primerwhich comprises a noncomplementary internal residue, and a secondnucleic acid primer to form a first and a second nucleoprotein primer;(c) contacting the first and second nucleoprotein primers to a nucleaseand to said double stranded target nucleic acid thereby forming a firstdouble stranded structure between said first nucleoprotein primer andsaid first strand of DNA at a first portion of said first strand and asecond double stranded structure between said second nucleoproteinprimer and said second strand of DNA at a second portion of said secondstrand such that the 3′ ends of said first extension blocked primer andsaid second nucleic acid primer are oriented toward each other on thesame double stranded target nucleic acid molecule, wherein said nucleasespecifically cleaves said modified noncomplementary internal residueonly after the formation of said first double stranded structure to forma first 5′ primer and a first 3′ extension blocked primer; (d) extendingthe 3′ end of said first 5′ primer and second nucleoprotein primer withone or more polymerases and dNTPs to generate an amplified targetnucleic acid molecule; (e) continuing the reaction through repetition of(c) and (d) until a desired degree of amplification is reached. Asexplained above, the first nucleic acid primer may be the firstextension blocked primer, said second nucleic acid primer, firstnucleoprotein primer or second nucleoprotein primer. Naturally, if thefirst primer is the first extension blocked primer, step (a) should beperformed in the presence of the nuclease. Further, it should be notedthat any RPA reaction which uses a single stranded nucleic acid targetDNA as a starting material will necessarily go through an intermediatestage where the target nucleic acid is double stranded and would beamplified by double stranded amplification.

Another aspect of the invention is directed to a primer for RPA which isan extension blocked primer of between 12 to 100 residues in length andwherein the primer comprises one or more modified internal residues.This primer may be any of the extension blocked primer, including anyvariants thereof, described anywhere in this application. Briefly, themodified internal residue is selected from the group consisting of auracil residue, an inosine residue, 8-oxoguanine, thymine glycol, anabasic site mimic and analogs thereof. The abasic site mimic may be atetrahydrofuran residue or a5′-O-Dimethoxytrityl-1′,2′-Dideoxyribose-3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite(commonly known as a “D-spacer”) and analogs thereof.

The primer is extension blocked and cannot be elongated by polymerase(e.g., Klenow fragment) and dNTP. Methods of blocking a primer fromextension are known and are also described in this disclosure. Briefly,the primer may have a blocked 3′ residue. The blocked 3′ residue may bea blocking moiety. The blocking moiety, which optionally may comprise adetectable label, may be attached to the 2′ or 3′ site of the 3′ mostresidue of the primer. For example, the blocked 3′ residue may be a2′3′-dideoxy nucleotide.

In another embodiment, the primer comprises one or more detectablelabels. The detectable label may be a fluorophore, an enzyme, aquencher, an enzyme inhibitor, a radioactive label, one member of abinding pair and a combination thereof. In a more preferred embodiment,the primer comprises both a fluorophore and a quencher. The quencher maybe close to the fluorophore to suppress the fluorescence of thefluorophore. For example, the separation between the fluorophore and thequencher may be 0 to 2 bases, 0 to 5 bases, 0 to 8 bases, 0 to 10 bases,3 to 5 bases, 6 to 8 bases, and 8 to 10 bases. In a preferredembodiment, the fluorophore and quencher are separated by a greaterdistance when the extension blocked primer is unhybridized (but attachedto recombinase and/or single stranded binding protein) than when theextension blocked primer is hybridized to the target nucleic acid. Thefluorophore and quencher may be any fluorophore and quencher known towork together including, but not limited to, the fluorophore andquenchers any of the flurorophores described in this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts experimental data showing that lengthening primersaccelerate reaction kinetics in the case of primers targeting a Bacillussubtilis genomic locus.

FIG. 2 . depicts experimental results showing only the longer (45-mer)and faster primers successfully amplify DNA to gel detectable levelsusing ethidium bromide stain at 25° C., 23° C., 20° C., and 17° C.

FIG. 3 . depicts amplification kinetics at 25° C. appear roughly halfthose at 37° C. This figure also shows that PEG levels influence bothrate and specificity (a primer artifact is increased at high PEGconcentrations).

FIG. 4 . shows that primers for the Human ApolipoproteinB locus, ApoB4and Apo300, demonstrate rapid kinetics when only 33 and 32 residuesrespectively in length, and reaction kinetics (at 37° C.) are notaccelerated by elongation.

FIG. 5 . shows that primers for the Human ApolipoproteinB locus, ApoB4and Apo300, demonstrate amplification at 25° C. regardless of whetherthe 3′ end is elongated.

FIG. 6 . shows that UNG inhibitor peptide from Bacillus phage can beused in combination with E. coli UNG for a carry-over contaminationsystem which avoids a need for thermal denaturation of UNG.

FIG. 7 . depicts experimental data showing (a) A real-time detectionprobe comprising a FAM fluorophore, (b) a deep dark quencher, (c) anabasic site mimic, and (d) a blocked 3′ end, provide excellentcharacteristics in RPA reactions for monitoring specific productaccumulation.

FIG. 8 . depicts the development of a third probe detection system.Fluorescence data may be best interpreted through a process ofnormalization and plotting the log of fluorescence.

FIG. 9 . depicts the use of reversibly blocked primers to gain highsignal to noise ratios for sandwich assays. RPA reactions configuredwith a blocked, splittable, probe active only after splitting by Nfoenzyme can be analyzed directly on lateral flow test strips.

FIG. 10 . depicts experimental results showing development of adual-probe amplification/detection system for the hospital superbugMRSA.

FIG. 11 . depicts real-time probe-based detection of control MSSA DNAsequences.

FIG. 12 depicts a schematic of an RPA process.

FIG. 13 depicts the use of specific antibodies to immobilize and detectcomplexes containing two antigenic labels on a flowstrip.

FIG. 14 shows polyacrylamide gel electrophoresis of RPA reactions usingprimers for the human Sry locus.

FIG. 15 shows agarose gel electrophoresis of RPA reactions using primersfor the human Apolipoprotein B locus.

FIG. 16 depicts an investigation of the minimum oligonucleotides sizenecessary to support RPA

DETAILED DESCRIPTION OF THE INVENTION

In RPA the isothermal amplification of specific DNA fragments isachieved by the binding of opposing oligonucleotide primers to templateDNA and their extension by a polymerase (FIG. 1A). Unlike PCR, whichrequires global melting of the target template, RPA employsrecombinase-primer complexes to scan double-stranded DNA and facilitatestrand exchange at cognate sites. The resulting structures arestabilized by single-stranded DNA binding proteins (SSBs) interactingwith the displaced template strand, thus preventing the ejection of theprimer by branch migration. Recombinase disassembly leaves the 3′-end ofthe oligonucleotide accessible to a strand displacing DNA polymerase inthis case the large fragment of B. subtilis Poll (Bsu) (See, Okazaki etal., 1964), and primer extension ensues. Exponential amplification isaccomplished by the cyclic repetition of this process.

In this disclosure, we showed a number of improvements over the basicRPA process. First, we found that with modifications to standardconditions, RPA may be performed efficiently at 25° C. or 30° C. Thesereaction temperatures allows for equipment-free RPA tests with resultsin under an hour.

Second, we improved the sensitivity and specificity of RPA reactions byusing DNA repair enzymes in the RPA reaction. In this study, we employeda wide spectrum of previously identified repair enzymes directly in RPAreactions to see if these enzymes would have an effect on RPA efficiencyand fidelity. We hypothesize that primer artifacts arise in RPAprincipally by errant extension of short-lived hairpin structures formedby the primers, or possibly by forming primer dimers (PCT ApplicationPCT/IB2005/001560 filed Apr. 11, 2005). Although such events arepresumably rare, the high concentration of oligonucleotide in areaction, typically of the order 10¹²-10¹³ molecules would tend topromote a significant degree of such events when the concentration oftarget template nucleic acid (i.e., the nucleic acid to be amplified) islow. It should be noted that these side reactions are distinct in naturefrom those often reported in PCR in which poorly-related sequences areamplified from complex DNA samples due to low fidelity of extension fromhybridization products in which only a limited number of 3′ residues arehomologous to parts of the sample DNA. In RPA we believe that theprimary recombinase-mediated pairing requires significant homology oversignificant regions, and rather that single-stranded DNA's are thespecies mainly sensitive to artifacts through snapback events occurringat the relatively low temperatures employed. Because of thisdistinction, methods for reducing primer artifacts in PCR do notnecessarily work in RPA reaction. This distinction is important tocomprehending the approach and mechanism described below for decreasingthe background noise generated in the system even in the absence of anytarget nucleic acids, and the way in which this increases sensitivity bydecreasing the competitive primer noise.

We disclose herein the use of primers deliberately modified with a3′-blocking group (with a biotin, ddC residue, or otherwise), andadditionally containing a roughly centrally positioned modified (orabsent) base. The internally positioned modification became a nucleasetarget for a repair endonuclease enzyme, which could split the primer togenerate two separate primers only if first paired to a target togenerate a stable duplex, and then secondarily processed by the enzyme.If one of the new daughter primers (i.e. the most relatively 5′positioned) possesses, or can subsequently be processed to possess, afree extendable 3′ hydroxyl group, then it could subsequently functionas a polymerase substrate. In contrast the daughter oligonucleotidepositioned relatively 3′ would retain the original blocking modificationand be unable to function as a polymerase substrate. A dependence onsplitting the oligonucleotide to form two duplex hybrids separated by anick or single-nucleotide gap adds noise reduction to the RPA system asthere is little or no opportunity for the un-split primer to beerroneously extended in transient fold-back structures due to thepresence of the 3′ blocking group. We demonstrate the utility of thisapproach to reduce primer noise here by showing that trace DNA samplescan be detected and discriminated from water merely by assessing whethertwo labeled DNA primers become physically linked. The possibility ofsuch simple assays presents RPA as a powerful tool in the development ofcheap, disposable, equipment-free DNA tests.

Finally we have adapted the above duplex-specific nuclease system to thedevelopment of proprietary real-time fluorescent probes. We anticipatedthat the design of effective fluorescent probes would be quite distinctin the RPA system in comparison to other described systems, such as inthe PCR method. Why is this? We identified two key areas of difference.First, the organization of the functional groups on the probe wouldlikely be necessarily different due to the extreme difference betweenRPA reaction environments and those of other amplification systems.Earlier work demonstrated that the RPA reaction environment wasfundamentally and critically distinct from that encountered in othernucleic acid amplification reactions. Saturating quantities ofsingle-stranded DNA binding protein and recombinase protein ensures thatoligonucleotides with non-modified backbones do not adopt a random coilstructure. DNA's are relatively ‘stretched out’ and rigid as theseproteins imbue the nucleoprotein filament with a filament length roughly1.5 times that of B-form DNA (Yang et al., 2001; Scheerhagen et al.,1985; Kuil M E et al., 1990). Consequently the supposition that probescovalently linked to fluorophores and quenchers distant in the primarysequence will still quench due to frequent random approach does not holdtrue. The second key area in which RPA probes were anticipated to bequite distinct form those in other described systems relates to theenzymes employed in probes processing. We discovered experimentally thatdescribed approaches using the 5′ exonuclease domain of Pol I classenzymes appeared incompatible with RPA (so-called ‘Taqman’ method),likely due to FLAP endonuclease activity of these enzymes (Kaiser etal., 1999). We further anticipated that other systems such as molecularbeacons or scorpion probes were similarly unlikely to be practical (dueto the instability of short duplex anchors in RPA conditions). Instead,we here show that it is possible to configure excellent real-time RPAprobes by placing fluorophore and quencher moieties close to one anotherseparated by a modified base that leads to backbone splitting only in aduplex context. This approach promises to add tremendous value to theRPA process as it brings the real-time quantitative detection andmultiplexing specifications into alignment with the currentstate-of-the-art using the other methods. Specifically it provides anapproach to assess absolute numbers of target nucleic acid molecules ina sample, to increase specificity and sensitivity to allow singlemolecule detection, and also to permit multiplex analysis of severaltargets. All of these properties can be attained using this methodwithout a need for gel electrophoresis, or other approaches requiringexperimental intervention, but rather reactions can be monitoredcontinuously and automatically by dedicated equipment. To illustrate thepower of combining the RPA process with these highly fidelitousdetection approaches we have developed an ultra-sensitive,internally-controlled, test for the hospital pathogen MRSA, a difficulttarget due to the complex and diverse nature of pathogenic strains, anda need for multiplexing.

Each aspect of the invention is described in more detail below:

Low Temperature RPA

RPA reactions operate optimally at about 37° C., reflecting thetemperature optimum of the enzymes involved in an RPA reaction. While37° C. is easily achieved in the laboratory, an RPA reaction that canfunction efficiently at 30° C. or 25° C. would increase the utility ofRPA and allow real time amplification under field conditions where a 37°C. incubate in not available.

To determine if primer length has an effect on RPA efficiency, RPAreactions were performed at 37° C. with primer pairs of differentlengths (FIG. 1 ). The results of the experiments, as shown in FIG. 1 ,shows that primer ‘rates’ can be enhanced by lengthening primers. PanelA of FIG. 1 shows the primer organization at the B. Subtilis locustargeted by BsA1 and BsB3 primers for RPA amplification. The primersBsA1 and BsB3 (30 and 31 residues respectively), or derivativescontaining extensions which retain appropriate homolog with the targetwhich were used in the RPA reactions. Panel B shows the results ofamplification kinetics monitored in a BIOTEK Flx-800 microplate readerwith heated stage set to 38° C. SYBR-green was employed to assess DNAaccumulation. Precise reaction conditions and component concentrationsare as follows: 10 copies/μl; 10 mM Mg acetate; 50 mM Tris pH 7.9; 100μM dNTPs; 600 ng/μl gp32; 120 ng/μl uvsX; 30 ng/μl uvsY; 300 nM oligos;5% Carbowax 20M; 1:50,000 SYBR green; 100 mM Pot. acetate; 20 mMPhosphocreatine; 100 ng/ml CK (creatin kinase); 3 mM ATP.

It is understood that the primers for any of the methods of theinvention may be made from DNA, RNA, PNA, LNA, morpholino backbonenucleic acid, phosphorothiorate backbone nucleic acid and a combinationthereof. Combinations thereof in this case refer to a single nucleicacid molecule which may contain one or more of one base connected to oneof more of another base. Preferred concentration of these molecules maybe in the range of between 25 nM to 1000 nM. In one preferredembodiment, the primers may contain a non-phosphate linkage between thetwo bases at its 3′ end and is resistant to 3′ to 5′ nuclease activity.

Our results show that there was a gradual increase in kinetic rate asthe primers were lengthened. In fact lengthening the primers from30/31-mers to 45-mers cut the amplification time to threshold detectionby about 10 minutes, from roughly 35 minutes to 25 minutes under theconditions used here (10 mM magnesium, 5% carbowax 20M). Based on theresults of this experiment, we conclude that primers with slow kineticsmay be enhanced by increasing primer length.

We also investigated whether primer length has an effect on RPAperformed in lower temperatures. RPA may not work at a lower temperaturefor at least two reasons. First, there can be a sudden and dramaticcessation of RPA reaction function below a certain temperature if, forexample, one of the components of the reaction cease to function below acertain temperature. For example, the carbowax may go through a phasetransition at a lower temperature and cease to function in the desiredfashion. Second, the reaction rate may simply slow progressively so thatdoubling times lengthen, a reflection of slower enzyme catalysis anddiffusion. In the second case, the primer ‘rate’ could be very importantbecause the reaction would possibly be ‘up-against-the-clock’ withregard to exhaustion of reaction components such as ATP.

To test our hypothesis, we attempted to amplify the same fragments as inFIG. 1 but at 25° C. The results, shown in FIG. 2 , indicate thatprimers with fast kinetics can amplify DNA at typical ambient (room)temperatures. The primers used in FIG. 1 were used to amplify a specificfragment from the B. subtilis genome. FIG. 2A shows the schematicarrangement of primers. FIG. 2B shows that only 45-mers amplify todetectable levels at 25° C. Conditions used were: 50 mM Tris pH 8.4, 100mM Potassium acetate, 10 mM Magnesium acetate, 2 mM DTT, 7.5% PEGcompound (Carbowax-20M), 3 mM ATP, 25 mM Phosphocreatine, 100 ng/μlcreatine kinase, 700 ng/μl gp32, 160 ng/μl uvsX, 40 ng/μl uvsY, 200 μMdNTPs, 300 nM each oligonucleotide. Reaction time, 90 minutes. Startcopy density 2 copies/μl, reaction volume 50 μl. FIG. 2C shows that only45-mers amplify DNA at 23° C., and amplification to detectable levelscan also occur at 20° C. and 17° C. when the 45-mer is used althoughprogressively less amplification product was recovered. Conditions used:50 mM Tris pH 8.4, 100 mM Potassium acetate, 14 mM Magnesium acetate, 2mM DTT, 7.5% PEG compound (Carbowax-20M), 3 mM ATP, 50 mMPhosphocreatine, 100 ng/μl creatine kinase, 650 ng/μl gp32, 125 ng/μluvsX, 40 ng/μl uvsY, 200 μM dNTPs, 300 nM each oligonucleotide. Reactiontime, 120 minutes. Start copy density 1 copy/μl, reaction volume 20 μl.

As seen in FIG. 2 , specific amplification of about 10¹⁰ fold observedeven at temperatures at low as 17° C. The time to detection was within 2hours. In the experiments performed at 23° C. or below only 20 copies ofgenomic DNA were added, and although some trace carry-over contaminationhad been in evidence from water controls (not shown), the attainment ofvisible product when using ethidium bromide stain (estimated 20 ngs at17° C.) suggests an amplification level of around 10⁹-fold, or 30cycles. Importantly high levels of ‘noise’ are not apparent, although wedid observe one additional fast-migrating extra band of unidentifiednature (quite possibly classical primer dimer, or single-stranded DNArelated to the product).

The kinetic behavior of the 45-mer primers at 25° C., under differentconcentrations of PEG, is shown in FIG. 3 . In FIG. 3 , the 45-merprimers used in FIGS. 1 and 2 were used to amplify a fragment of the B.subtilis genome at 25° C. FIG. 3A shows the arrangement of the primerpair used. FIG. 3B shows agarose gel electrophoresis and ethidiumbromide staining of samples at reaction endpoint. The expected band (*)is accompanied by an additional band at higher PEG concentrations (#).FIG. 3C shows the kinetics of the amplification reaction monitored usingSYBR-green. Conditions used was as follows: 50 mM Tris pH 8.4, 100 mMPotassium acetate, 10 mM Magnesium acetate, 2 mM DTT, PEG compound(Carbowax-20M) as indicated, 3 mM ATP, 25 mM Phosphocreatine, 100 ng/μlcreatine kinase, 650 ng/μl gp32, 160 ng/μl uvsX, 40 ng/μl uvsY, 200 μMdNTPs, 300 nM each oligonucleotide, SYBR-green 1:50,000 from stock.Reaction time, 120 minutes. Start copy density 10 copy/μl, reactionvolume 50 μl.

The lack of a signal in the 4% lane is possibly due to experimentalerror. The results show that higher PEG concentrations can acceleratekinetics up to a point, and then some inhibition in rate and overallreaction behavior/outcome is observed. In this case 7% or 8% PEG wereoptimal for maximizing the amount of amplified nucleic acids of thecorrect length. When the PEG concentrations are higher, there isprogressive domination of the faster-migrating anomalous band. In thepresence of 8% PEG detection was observed by about 37 minutes at 25° C.,which corresponds to a doubling time of around 1 minute 25 seconds. At5% PEG detection was made at about 54 minutes (corresponding to a 2minutes doubling time). This reaction at 25° C. is about half as fastasthe experiment shown in FIG. 1 (detection time of 27 minutes anddoubling time of 1 minute. Based on this, we estimate RPA reaction rateshalve with roughly every 10° C. drop in temperature. Further, due tolimited pools of reagents such as ATP, detectable product formation maybe limited regardless of incubation time depending on the temperature,activity of the primers, and product length. Our results suggest thateffective low temperature RPA would be improved with primers that showfast kinetics, and which are not rate limiting in the reaction.

The experiment of FIG. 3 was repeated using primers targeting the humanApolipoprotein B gene and the results are shown in FIG. 4 . FIG. 4Ashows the arrangement of primers targeting the human Apolipoprotein Blocus. Three primer pairs were used as shown, and overlapping primersshared a common 5′ extremity but different 3′ ends. (B) Kinetics ofamplification at 38° C. Reactions with the indicated primer pairs weremonitored in real-time using SYBR-green dye. Start target copy numberswere either 1 copy/μl or 100 copies/μl of human DNA. Reaction conditionswere as follows: 50 mM Tris pH 7.9, 100 mM Potassium acetate, 10 mMMagnesium acetate, 2 mM DTT, 5% PEG compound (Carbowax-20M), 3 mM ATP,25 mM Phosphocreatine, 100 ng/μl creatine kinase, 600 ng/μl gp32, 120ng/μl uvsX, 30 ng/μl uvsY, 100 μM dNTPs, 300 nM each oligonucleotide,SYBR-green 1:50,000 from stock. Reaction time, 60 minutes. Reactionvolume 50 μl.

Primers for the Human Apolipoprotein B locus show rapid kinetics withoutprimer elongation. In this case kinetic studies using SYBR-greenrevealed that no rate increase was found with longer RPA primers. Itappears that the ApoB4 and Apo300 primers used here, even when short,possess high rate behavior to the extent that they are not the ratelimiting factor in the reaction. Presumably, in this reaction,polymerase rate is now the main rate-limiting part of the reaction andmore active (longer) primers cannot achieve an overall speed benefit.Consistent with our hypothesis, we find that all of the Apolipoprotein Bprimers generate the expected product at 25° C. (FIG. 5 ). FIG. 5A isthe same as FIG. 4A in that it shows the arrangement of the primersused. FIG. 5B shows gel electrophoresis of RPA reactions performed at25° C. using the indicated primer pairs. Copy numbers of zero or 10copies/μl were tested in each case. Conditions used were as in FIG. 4with the exception of the omission of SYBR-green. In this case, noartifact band is seen—supporting the idea that RPA reactions do notsignificantly suffer from ‘noise’ at reduced temperatures.

Contamination Control Using UNG Inhibitor from Bacteriophage PBS2

RPA reactions are compatible with the use of dUTP as a method to controlcarry-over contamination. One caveat with the earlier experimental datais that in order to initiate the reaction the uracil glycosylase enzymehad to be heat inactivated. This poses two incompatibility issues withRPA. First, heat inactivation would also inactivate complete RPAreactions because RPA reagents are not heat stable. Second, heatinactivation is inconsistent with one goal of RPA—the avoidance ofthermal cycling.

Because of the reasons above, we set to investigate another technicalroute to implement contamination control. It is known that the Bacillussubtilis phages PBS1 (See, Savva and Pearl, 1995) and PBS2 (See, Wang,Z. and Mosbaugh, D. W. (1989)) possess a specific small peptideinhibitor of E. coli and B. subtilis uracil-DNA glycosylase (Wang andMosbaugh, 1988). They require a highly effective system as their own DNAis synthesized using dUTP rather than dTTP. We cloned the PBS2 DNAsequencing encoding the inhibitor peptide and expressed it in E. coliwith a C-terminal hexahistidine tag. We also cloned the E. coli uracilglycosylase gene and expressed it with a C-terminal hexahistidine. Weused these protein preparations to test whether a carry-overcontamination system could be employed with them. FIG. 6 shows anexample of experiments performed which validate that such an approach.In FIG. 6 , the start target copy numbers of the template were 800copies of human DNA where used. Reaction conditions were as follows: 50mM Tris pH 8.4, 100 mM Potassium acetate, 10 mM Magnesium acetate, 2 mMDTT, 5% PEG compound (Carbowax-20M), 3 mM ATP, 25 mM Phosphocreatine,100 ng/μl creatine kinase, 600 ng/μl gp32, 125 ng/μl uvsX, 30 ng/μluvsY, 100 μM dNTPs, 300 nM each oligonucleotide (SRY8 and SRY9 primers).Reaction time, 75 minutes. Reaction volume 50 μl. Where used E. coli UNGwas used at 150 ng/μl, and UNG inhibitor was used at 140 ng/μl.Contamination was genuine carry-over contamination present for thisamplicon in the laboratory liquid-handling equipment. Reactions wereestablished with all amplification components apart from the polymerase.Reactions 1-4 carried genomic template DNA, reactions 5 and 6 containedonly contaminating material. The samples were treated for 5 minutes withUNG in samples 2, 3, 4, and 6. In samples 2, 4, and 6 UNG inhibitor wasadded after 5 minutes. In all cases after the 5 minute incubationperiod, with or without UNG and with or without subsequent addition ofUNG inhibitor, polymerase was added to initiate DNA synthesis. In thisexperiment we show the following: (1) that E. coli UNG will inhibit RPAreactions containing dUTP substrate, (2) that co-inclusion of theinhibitor peptide overcomes this inhibition, (3) that dUTP-containingcontaminants can be suppressed from generating amplicons if firsttreated with E. coli UNG and then with the inhibitor, but that bona fidetemplates are still effective. Under the conditions used we have seensome evidence of some decrease in robustness/product level when UNG waspresent in the reaction. We anticipate however that the system may beconfigured more optimally.

Fluorescent Real-Time Probes for RPA Reactions

Many possible applications of the RPA process in detecting DNA (or RNA)sequences would benefit from being applied in a real-time format. RPAhas already been shown to be effective when combined with minor groovebinding dyes such as SYBR-green (PCT Application PCT/IB2005/001560 filedApr. 11, 2005). However there may be potential limitations of using suchgeneral indicators of DNA accumulation to assess reaction behavior.First, there is no capacity for multiplexing amplification reactions asthe dyes cannot discriminate between the various products formed. Inmany clinical tests, for example, there would be a need to include aninternal amplification control to exclude false negatives. Second, RPAreactions are similar to most other DNA amplification processes insofaras even when no target is present in a sample, some DNA synthesis willeventually ensue. Consequently may be difficult or impossible todiscriminate between the presences of a few copies of target nucleicacid or no copies of a nucleic acid based on current methods offlorescent detection.

In response to these issues we have developed a proprietaryfluorescence-based probe system to monitor RPA reactions. Weinvestigated using the 5′-3′ nuclease associated with the polymerases ofthe E. coli Pol I class. This nuclease is used in a fluorescent probemethodology for PCR known as the 5′nuclease, or ‘Taqman’, assay. Wefound that both Bacillus subtilis Pol I retaining the 5′-3′ nucleasedomain and the E. coli Poll enzyme would not support RPA reactions. Onreflection we believe this arises because these nucleases arestructural/functional homologs of the FEN1 FLAP endonuclease family andmost likely are structure-specific endonucleases (Kaiser et al.). Wesuppose these enzymes progressively digest the displaced strand duringthe strand-displacement synthesis thus inhibiting DNA amplification.

We focused our attentions particularly on the E. coli glycosylaseenzymes and AP endonucleases involved in DNA repair known as fpg, Nth,Nfo, and more recently E. coli exonuclease III. Importantly theseenzymes will only remove damaged bases and/or nick DNA backbones atpositions in which base modifications have occurred and, critically, inthe context of duplex DNA. All of these enzymes are able to cleave suchappropriate duplex DNA molecules with high specificity in the RPAenvironment (see application). Test probes were utilized that containeda modified base within the body of the oligonucleotide (8-oxoguanine,thymine glycol, or abasic site mimic respectively) and an additionaldistinct elongation blocking group on the 3′ end (provided by a3′-dR-biotin). Despite obvious promise for all of these enzymes, andpotentially other repair/processing enzymes, we focused on the behaviorof the E. coli Nfo and exonuclease III enzymes for the followingreasons. First, we observed when testing fpg, Nth, and Nfo proteins thatthe degree of successful probe processing was highest for the probecontaining a tetrahydrofuran residue (THF—an abasic site mimic), andprocessed by Nfo. Second, because Nfo, and the functionally similar E.coli exonuclease III, split the oligonucleotide into two smalleroligonucleotides separated by a single nucleotide gap, in which the new3′ end that is formed can be elongated by a strand displacing polymerasethat can initiate at nicks. This property endows the THF/Nfo orTHF/exonuclease III processing system with a wealth of applicationopportunities that extend beyond application to fluorescent probeprocessing. (Note that other abasic site mimics, or true abasic sitesmight also be employed).

A previous report has also illustrated a potential use of employing anabasic, or other blocking residue, in the context of an amplificationprocess, with the preferred intention to remove the residue in thecontext of PCR or LCR reactions using a thermostable nuclease (U.S. Pat.No. 5,792,607, referred to herein as the '607 patent). However theapproach we used is distinct from that of the '607 patent. In the '607patent, an abasic site is described as one member of a broader selectionof modifying groups, to be positioned preferentially at the 3′ end ofthe intended amplification oligonucleotide, and designed to serve as areversible 3′ sugar modifying group by effectively preventing substraterecognition or catalysis by the polymerase. The intention is to decreasethe propensity of the amplification system to amplify unintended targetsin sample DNA because of the tendency of PCR and LCR techniques to form,albeit at reduced frequency, hybrids with sequences sharing limitedhomology to the 3′-region of oligonucleotide primers. Furthermore it isintended, critically, in the '607 patent that this modificationpreventing substrate recognition be specifically corrected in atarget-dependent fashion. Such an activity might be performed by theactivity of an agent such as endonuclease IV which can ‘polish’ groupsfrom a 3′ sugar residue. However, quite distinctly, in the processdescribed herein the THF residue does not serve as anelongation-blocking modification agent to the 3′ sugar that prevents theinitial oligonucleotide/template hybrid being recognized as a bona fidesubstrate. Indeed the THF residue, instead of being located at the very3′ end of an oligonucleotide, is positioned within the body of theoligonucleotide, away from the substrate target of the polymerase (i.e.the 3′ end region of the hybridized primer on the template DNA). In thisdisclosure the principal motivation is to prevent noise arising fromprimer fold-back artifacts. Thus, instead, herein the processing of theTHF residue by an endonuclease activity leads to incision of theoligonucleotide backbone in the context of a bona fide duplex in adistinct event from ‘correction’ of the modification that preventspolymerase substrate recognition. We also describe herein 3′ terminalelongation-blocking modifications, however these are not the ‘corrected’modification in this case, and are not necessarily removed from3′-terminal nucleotides as in the '607 patent. Instead, in the casedescribed here we would employ two separable entities, a non-corrected3′-blocking group, and a centrally located abasic-like residue which canbe incised by an AP endonuclease to generate a nicked structure and twoindependent daughter annealed primers, only one of whom is a polymerasesubstrate.

FIG. 7 shows the results of an experiment in which a fluorescent sensingprobe has been employed to assay for the accumulation of a specificamplicon in an RPA reaction. FIG. 7A shows a schematic structure of theprobe. The probe has internal base-labeled fluorophore and quencher(fluorescein and deep dark quencher II) which were incorporated duringsynthesis by using commercially available (Glen Research, Sterling, Va.,USA) fluorescein-dT or DDQ2-dT amidites.

A THF residues was inserted at a nucleotide position between thesemodified bases. The probe was blocked by the presence of a 3′-dR-biotingroup. FIG. 7B shows the probe sequence which is:

(SEQ ID NO: 8) 5′-catgattggatgaataagctgcagc (dTfluoro) g (THF) t(dT-DDQ1)aaaggaaactta-dRbiotin-3′

The probe is homologous to part of the Bacillus subtilis SpoOB locuscontained within an amplicon generated by primers J1 and K2. Thefluorophore and quencher were designed to be on T residues in thesequence so that they could be incorporated directly on commerciallyavailable amidites. FIG. 7C shows the amplification and probe cleavagekinetics as monitored by fluorescence increase. Amplification reactionswere established with varying concentrations of target Bacillus subtilisgenomic DNA. Reactions were established on ice and then incubated in aBIOTEK Flx800 microplate reader with stage set at 38° C. Amplificationconditions are as follows: Start target copy numbers were as indicated.Reaction conditions: 50 mM Tris pH 7.9, 100 mM Potassium acetate, 12 mMMagnesium acetate, 2 mM DTT, 5% PEG compound (Carbowax-20M), 3 mM ATP,25 mM Phosphocreatine, 100 ng/μl creatine kinase, 900 ng/μl gp32, 120ng/μl uvsX, 30 ng/μl uvsY, 180 ng/μl Nfo, 100 μM dNTPs, 450 nM of K2primer, 150 nM J1 primer, 100 nM probe. Reaction time, 60 minutes.Reaction volume 20 μl.

The sensing probe was designed to possess a fluorophore and quencherseparated by (a) less than 10 bases (to ensure efficient quenching) and(b) a cleavable site (THF residue). In this case the primary ampliconwas generated using the primers J1 and K2 to amplify a fragment from theBacillus subtilis SpoOB locus. RPA reactions were modified from ourusual conditions in the following manner. First the probe was included,whose overall structure and sequence is shown in the lower part of thefigure. Second the amplification primers were biased in concentration sothat there was a relative excess of the amplification primer opposingthe probe in order that there might be a steady-state excess ofcomplementary sequences to the probe. Finally the Nfo enzyme wasincluded in the reaction. Reactions were performed in 20 microlitervolumes in a standard 384-well plate and fluorescence monitored usingexcitation/detection filters of 485/525 in a BIO-TEK Flx800 platereader. We observed that there was a template-dependent increase influorescence. The time at which accumulation begins was dependent on thecopy number, as was the level of total fluorescence at the end of theperiod of reaction monitoring at one hour.

In FIG. 8 this experiment was repeated. FIG. 8A shows the rawfluorescence data while FIG. 8B shows normalized fluorescent signals.The fluorescence signal present in the water control at any given timewas subtracted from all other sample fluorescence signals. All sampleswere normalized to one another by adjusting them to a common baselinebased on the period prior to measurable fluorescence rise. In FIG. 8C,the log of the normalized fluorescence data was plotted and in FIG. 8Dthe time of threshold crossing of the fluorescence signal (set to about2.6) was plotted against start copy number.

In this case we have shown the result of normalizing the samples againstthe signal in the water control, and then the results of plotting thelogarithm of the normalized fluorescence signal. We set a fluorescencesignal of 2.5 or above as constituting a positive signal. Note that itis easy to distinguish the low copy samples from water in contrast tothe situation usually observed when using SYBR-green. The slightfluorescence increase in the water sample is almost certainly due toslight carry-over contamination associated with this particular ampliconwhich has been handled widely in the laboratory.

With respect to the quenchers of this disclosure, it is understood thata quencher need not be a fluorophore. A non-fluorescent chromophore canbe used that overlaps with the donor's emission (a dark quencher). Insuch a case, the transferred energy is dissipated as heat.

High efficiency dark quenchers, such as Dark Quencher 1, Dark Quencher 2and Black Hole Quencher1 and Black Hole Quencher 2 are known andcommercially available (Biosearch Technologies, Inc., Novato, Calif.).As is known in the art, the high quenching efficiency and lack of nativefluorescence of the dark quencher allows attachment of a fluorophore anda quencher on one oligonucleotide and ensures that such anoligonucleotide does not fluoresce when it is in solution.

Suitable fluorophores and quenchers for use with the polynucleotides ofthe present invention can be readily determined by one skilled in theart (see also, Tgayi et al., Nature Biotechnol. 16:49-53 (1998); Marraset al., Genet. Anal.: Biomolec. Eng. 14:151-156 (1999)). Manyfluorophores and quenchers are available commercially, for example fromMolecular Probes (Eugene, Oreg.) or Biosearch Technologies, Inc.(Novato, Calif.). Examples of fluorophores that can be used in thepresent invention include, but are not limited to, fluorescein andfluorescein derivatives such as FAM, VIC, and JOE,5-(2′-aminoethyl)aminonaphthalene-1-sulphonic acid (EDANS), coumarin andcoumarin derivatives, Lucifer yellow, NED, Texas red,tetramethylrhodamine, tetrachloro-6-carboxyfluoroscein, 5carboxyrhodamine, cyanine dyes and the like. Quenchers include, but arenot limited to, DABSYL, 4′-(4-dimethylaminophenylazo)benzoic acid(DABCYL), 4-dimethylaminophenylazophenyl-4′-maleimide (DABMI),tetramethylrhodamine, carboxytetramethylrhodamine (TAMRA), Black HoleQuencher, Dark Quencher 1, and Dark Quencher 2. Methods of couplingfluorophores and quenchers to nucleic acids are well-known in the art.

We have successfully implemented a fluorescent probe system in the RPAreaction environment and established the general structure of probes.With this knowledge it should be easy to develop probes to detect anyamplicon, and by judicious selection of alternate fluorophores, tomultiplex more than one amplification at once. To demonstrate this wehave developed a multiplex test for the antibiotic-resistant S. aureuspathogen known in the United Kingdom as methicillin-resistantStaphylococcus aureus, or MRSA for short.

The Detection of Methicillin-Resistant Staphylococcus aureus

MRSA comprises a collection of Staphylococcus aureus strains which havedeveloped antibiotic resistance by integration of a resistance cassette,the mecA cassette, at a specific location in the S. aureus genome. Whilethe same general genomic integration site is always used, the preciseintegration site junctions and orientation of the cassettes can vary.Despite this variation, independent isolates can be segregated into alimited number of general groups with representative integrationstructures. In addition to this complexity, further difficulties arisedue to the existence of base polymorphisms between strains which cancompromise the effectiveness of amplification primers and probes. TheMRSA pathogen thus represents a complex target because in order tocapture over 90% of the strains commonly found in clinical specimens ina single test it is necessary to accommodate detection of threestructurally distinct variations of the mecA resistance cassetteintegration locus, and account for some common polymorphisms.Additionally, it is necessary that the amplicon spans one arm of theintegration cassette to ensure that any mecA sequences amplified are inthe context of the S. aureus genome, and were not present in anunrelated bacterium.

In order to configure an RPA test for over 90% of common MRSA strains,we developed a primer design strategy which is illustrated in FIG. 10 .FIG. 10 depicts the real-time detection of MRSA alleles in a multiplextest environment. FIG. 10A is a schematic of the RPA probe principle.Signal generation depends on probe cutting by double-strand specificNfo. FIG. 10B depicts an arrangement of primers and probes relative tothe targets used in 2C-F and 3C. A PCR fragment that fused an unrelatedsequence to the target sites sccIII and orfX served as internal control.FIG. 10C shows probe signal of RPA reactions using the primer setorfX/sccIII. MRSAIII DNA at 10⁴ (black, reactions 1-3), 10³ (red, 4-6),100 (yellow, 7-9), 10 (green, 10-12) or 2 copies (purple, 13-17) orwater (blue, 18-20) served as template. FIG. 10D shows a plot of theonset time of amplification (defined as passing the 2.5 threshold) inreactions 1-12 in 2C against the logarithm of the template copy numberreveals a linear relationship. (E) A multiplex RPA approach enablesdetection of different MRSA alleles and an internal control in the samereaction. MRSAI (green), MRSAII (dark blue), MRSAIII DNA (red) at 10copies or MSSA DNA at 10⁴ copies (blue, negative control) or water(yellow, turquoise) served as a template (in triplicate for eachtemplate condition). (F) Detection of the 50 copies of internal controlDNA included in the reactions in 2E. A negative control contained water(turquoise). The RPA reactions were performed as follows: Real-time RPAwas performed in a plate-reader (BioTek Flx-800) in the presence offluorophore/quencher probes. Reactions were performed at 37° C. for 90minutes. Conditions were 50 mM Tris (pH 7.9), 100 mM Potassium-acetate,14 mM Magnesium-acetate, 2 mM DTT, 5.5% Carbowax20M, 200 μM dNTPs, 3 mMATP, 50 mM Phosphocreatine, 100 ng/μl Creatine-kinase, 20 ng/μl Bsu.Concentrations of gp32/uxsX/uvsY (in ng/ul) were 900/120/30. Primerswere employed at 265 nM sccI/II, 265 nM sccIII, 70 nM orfX. Reactionvolumes were 20 μl.

Three probes were employed:

SATamral1 (SEQ ID NO: 9) 5′-tgttaattga acaagtgtac agagcatt (T)a(H)ga(q1)tatgcgtgga g-Biotin-3′ SATamra2 (SEQ ID NO: 10)5′-tgttaattga gcaagtgtat agagcatt (T)a(H)ga(q2) tatgcgtgga g-Biotin-3′BSF1c (SEQ ID NO: 11) 5′-catgattgga tgaataagct gcagc (F)g(H)t(q3)aaaggaaact ta-Biotin-3′

Here (T) is dT-TAMRA, (F) is dT-Fluorescein, (H) is THF, (q1) isdT-BHQ1, (q2) is dT-BHQ2, (q3) is dT-DDQ1. Probes were employed at 60 nMSATamra1 (MRSAIII experiment) or at 45 nM SATamra1, 45 nM SATamra2, 60nM BSFlc (multiplex experiment). Nfo was used at 200 ng/ul.Excitation/detection was at 485/525 nm (SybrGreenI, BSFlc) or 530/575 nm(SATamra1/2). Measurements were taken every 30 sec or 45 sec (multiplexexperiment). Fluorescence probe data were normalized against watercontrol and pre-amplification baseline adjusted. The logarithm of theread-out was plotted against reaction time.

Briefly, a single primer was designed to recognize the S. aureus genomicDNA outside of the integration cassette region, and is termed orfX. Twofurther primers specific to the mec cassette were designed, and one ofthese (scc I/II) can be used to amplify the locus from two of the strainvariants, while the second (scc III) amplified the locus from the thirdvariant. Two probes for the amplicons are used, differing in tworesidues to account for common single nucleotide polymorphisms. Boththese MRSA probes use TAMRA as fluorophore. Finally a control ampliconis included in the reaction which comprises a unique segment of anunrelated B. subtilis genomic DNA fragment fused to the orfx and sccIIIprimers, and a third probe may be used to sense this amplicon (BSFlc,and this is the same probe used in the experiments in FIG. 7 , containsa fluorescein and deep dark quencher I). FIG. 10 part A illustrates onceagain the strategy for developing increased fluorescence in the reactionby processing of probes forming hybrids with amplicons. In Part Cdetection of one MRSA genomic DNA template is demonstrated over a wideconcentration range in a non-multiplexed environment. Part E shows theresults of an experiment in which (approximately) 10 copies of each ofthe three types of MRSA were separately detected using a single reactionmastermix. In part F the signal generated by the control sequence in thefluorescein channel is shown, and we can see that all those samplescontaining control DNA score positive.

Included in these experiments are control reactions containingrelatively high concentrations of (10⁴ copies) of non-resistant S.aureus DNA. Satisfyingly, these samples do not score positive indicatinga strict requirement for both S. aureus sequences as well as the mecAcassette. To ensure that this control DNA was functional and that thecopy concentration was as indicated, the DNA was used in controlreactions employing a combination of the orfx primer and a second S.aureus specific primer termed mssa. In this case the same probes may beemployed as the probes recognize common sections of the S. aureusgenome. In FIG. 11 we can observe the results of an experiment performedwith these non-resistant strain specific primers, and see how thecontrol MSSA DNA is indeed effective, and shows appropriate response ofthe quantitative analysis to copy number. FIG. 11 depicts the detectionof MSSA DNA in a real-time quantitative RPA reaction. Probe signal ofRPA reactions using the primer set orfX/mssa and probe SATamra2. FIG.11A depicts measurement of MSSA DNA at 10⁴ (black, reactions 1-3), 10³(red, 4-6), 100 (yellow, 7-9), 10 (green, 10-12) or 2 copies (purple,13-17) or MRSAI DNA at 10⁴ copies (grey, reactions 18-20) or water(blue, 21-23) served as template. Reaction conditions were 50 mM Tris(pH 7.9), 100 mM Potassium-acetate, 14 mM Magnesium-acetate, 2 mM DTT,200 μM dNTPs, 3 mM ATP, 20 mM Phosphocreatine, 100 ng/μlCreatine-kinase, 5% Carbowax20M, 900 ng/μl gp32, 120 ng/μl uvsX, 30ng/μl uvsY and 20 ng/μl Bsu. Oligonucleotides were employed at 500 nMmssa, 100 nM orfX and 60 nM SATamra2. Whilst the MSSA target isamplified even at very low concentrations, the negative control (MRSAI)does not generate a signal. FIG. 11B depicts a plot of the onset time ofamplification (defined as passing the 2.5 threshold) in reactions 1-12against the logarithm of the template copy number reveals a linearrelationship.

Detection of Trace Nucleic Acids by Association of Primers FollowingEnzymatic Generation of an Extendable 3′ End

RPA is ideally suited to the development of portable equipment-free, orequipment-light, DNA tests. However such tests would ideally employcheap, easy-to-use, approaches to determine whether amplification hasoccurred. Traditionally gel electrophoresis is used to assess whether aproduct of a defined size has accumulated. Alternatively fluorescentprobes may be employed. In either case significant hardware is requiredto perform the analysis and this prevents the test being used byend-users lacking appropriate equipment.

Other approaches may be used to determine whether or not DNAamplification has occurred. One convenient hardware-free approach is toperform a sandwich assay in which the presence of an amplicon isassessed by interrogating whether two labeled gene-specific primers havebecome associated in a common DNA duplex. This can be achieved bylabeling one amplification primer with a label, such as biotin, and anopposing primer with a second label, such as FAM. A variety ofapproaches can be employed to determine whether the two labeled primersbecome associated. For example in a conventional lateral flow stripassay (see for example patent EP0810436A1), two antibodies (or othermoiety such as streptavidin that binds with high affinity to one of theoligonucleotide labels) are employed. One antibody would be immobilizedon a flow membrane in a line or spot. The other is coupled to visibleparticles such as colloidal gold, latex particles, or similar. When thesample, in this case a diluted or undiluted amplification reaction, isapplied to a sample pad in which the antibody-coupled visible particlesare pre-deposited, the visible particles become stably associated withone of the labeled oligonucleotides. The entire sample then moves bycapillary action up the membrane and as it flows the other labeledprimer becomes ‘caught’ on the immobilized antibody. If the labeledprimers are not co-associated in a duplex then the antibodies ‘caught’on the membrane are not associated with the visible particles associatedwith the other primer. If, however, they are associated as a consequenceof amplification then the visible particles also become trapped on theline or spot, and a visible signal accumulates. Other approaches toassess for association of primers can be configured.

One problem with simple association assays, such as sandwich assays, isthe requirement that the primers do not associate unless bona fideamplification of the desired target has occurred. Any undesiredassociation will lead to a false positive signal. However such aclean-cut situation is rarely the case with most amplification methods,particularly when the target is not abundant. For example primer dimers,or other artifacts, tend to accumulate to some extent in the PCR methodregardless of optimization. RPA also suffers from the accumulation ofprimer-related artifacts as detailed earlier, and these are likely tointerfere with the direct combination of RPA with such simple read-outs.Indeed this general problem may underpin part of the reason thatsandwich assays have not been broadly implemented in currently availablehigh sensitivity/specificity DNA tests. Those commercially availablelateral flow systems marketed to assess PCR product accumulation areinconvenient, requiring a final step of hybridizing an additional probeprimer to the product after the reaction has been performed in order toavoid aberrant co-association of primers through DNA synthesis (e.g. TheGenline Chlamydia Direct test strip from Milenia).

We have configured RPA reactions to permit easy assessment of bona fidetarget amplification by direct addition to lateral flow strips, orpotentially by other similar methods. To attain a clean distinctionbetween positive and negative samples we have employed a labeled primerwhich is split by the E. coli Nfo or exonuclease III enzymes to generatetwo primers, one of which may be elongated. This is attained by blockingthe 3′ end of the oligonucleotide, and separately incorporating a THFresidue or product of employing a5′-O-Dimethoxytrityl-1′,2′-Dideoxyribose-3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramiditeduring oligonucleotide synthesis, referred to herein as “D-spacer”available from Glen Research, Sterling, Va., USA) within theoligonucleotide to act as a splitting target for the enzyme. Thedependence on formation of a stable duplex before the Nfo or exonucleaseIII enzymes will incise/split the primer ensures that aberrantassociation of this primer with the other labeled opposing primer doesnot occur, or is so infrequent as to fall below threshold of detection.

FIG. 9 shows data from experiment in which DNA from amethicillin-resistant S. aureus strain (EMRSA 16 strain containing themec2 cassette), or from a non-resistant reference strain (MSSA) has beensubjected to amplification in the presence of 3 primers. This experimentshows that a high signal to noise ratio amplification strategy suitablefor lateral flow assays or other simple sandwich detection schemes isfeasible. FIG. 9A shows a schematic of the arrangement of primers. Theleft-most primer, and the probe, recognize sequences present in the S.aureus genome, and similarly present in the S. aureus MSSA referencestrain as well as the MRSA16 strain which contains a downstream mecIIcassette insert. The right-most amplification primer is specific forsequences in the mecII cassette and is not found in the non-resistant S.aureus genome. The right-most primer is 5′-labelled with a biotinmoiety, while the probe is labeled with a 5′-FAM moiety. The probe isblocked with 3′ ddC, and contains an internal THF residue. In FIG. 9B,amplification reactions were established with the following conditions:50 mM Tris pH 7.9, 100 mM Potassium acetate, 14 mM Magnesium acetate, 2mM DTT, 5% PEG compound (Carbowax-20M), 3 mM ATP, 25 mM Phosphocreatine,100 ng/μl creatine kinase, 600 ng/μl gp32, 125 ng/μl uvsX, 30 ng/μluvsY, 270 ng/μl Nfo, 100 μM dNTPs, 100 nM of ORFX45b primer, 100 nMsccII-35-2-bio primer, 50 nM probe ORFXprobe2. Reaction time, 60minutes. Reaction volume 30 μl. Reaction temperature 37° C. Copy numberswere 1000 copies of MSSA DNA or 1000 copies of MRSA16 DNA, or water.After 60 minutes 1 μl of the reaction was diluted with 5 μl of PBS/3%Tween-20, and applied to the sample pad of a commercial lateral flowtest strip from Milenia using 100 μl of PBS/3% Tween-20 (Mileniaproduct: Genline hybrid-detect MGHD1).

In this case 2 of the primers act as the main amplification primer pair,and a third acts as a probe. The probe contains a 3′ blocking group anda separate internal THF residue to act as a splitting target, as well asa FAM label at the 5′ end. The probe opposes one of the mainamplification primers which is labeled with a biotin residue. Only if abona fide amplicon accumulates will the probe form stable hybrids thatare nicked/split by Nfo, elongated, and thus associate the 2 labeledprimers. The results of an experiment are shown in which RPAamplifications established in this way were performed on DNA from theresistant and non-resistant strains. A small quantity of the reaction (1μl) was then mixed with 5 μl of lateral flow running buffer (Phosphatebuffered saline with 3% Tween-20) and directly applied to a commerciallateral flow strip (Milenia-germany). After about 1-2 minutes the stripswere assessed for signal, and a photograph was taken. The test clearlydistinguishes positive from negative.

Other processing enzymes might be employed in such approaches. Inparticular the E. coli fpg, Nth, and exonuclease III enzymes, homologsfrom other phyla, base mismatch repair enzymes such as E. coli MutY,MutS and MutM, E. coli MUG, Human MUG, Ogg1, and the vertebrate Nei-like(Neil) glycosylases. Any combination of the above repair enzymes mightalso be employed. In particular note that E. coli Nfo (endonuclease IV),and E. coli exonuclease III, possess phosphodiesterase activities andare capable of processing the non-extendable 3′ ends of nicked productsof the other glycosylase/lyases to extendable 3′-hydroxyl residues.

All patents, patent applications and references, cited anywhere in thisdisclosure, are incorporated by reference in their entirety.

The invention will now be described further by way of examples. Theexamples are illustrative of the invention and are not intended to limitit in any way.

EXAMPLE Example 1 Nucleic Acid Sequences

Proteins and DNA

Coding sequences for uvsx, uvsy, gp32, Bsu and Nfo were amplified fromgenomic DNA (DSMZ, Germany), fused to hexahistidine-tags (N-terminal foruvsY, Bsu and Nfo, C-terminal for uvsX and gp32) and cloned intosuitable expression vectors. Overexpression and purification was done bystandard protocols using Nickel-NTA resin (Qiagen). S. aureus alleleswere EMRSA-3 (SCCmec type I; MRSAI), EMRSA-16 (MRSAII), EMRSA-1(MRSAIII) and wild-type MSSA. See additional sequence informationprovided below.

Primer sequences Human locus ApoB (product size experiment SI): Apo700tggtaaacgg aagtctggca gggtgattct cg (SEQ ID NO: 12) Apo800caattgtgtg tgagatgtgg ggaagctgga at (SEQ ID NO: 13) Apo900gaggtggttc cattccctat gtcagcattt gc (SEQ ID NO: 14) Apo1000gggtttgaga gttgtgcatt tgcttgaaaa tc (SEQ ID NO: 15)Human loci for STR markers (STR experiment and primer size experiment, SI):CSF1PO 5′ gttgctaacc accctgtgtc tcagttttcc tac (SEQ ID NO: 16) CSF1PO 3′agactcttcc acacaccact ggccatcttc agc (SEQ ID NO: 17) D7S820 5′gaacacttgt catagtttag aacgaactaa cg (SEQ ID NO: 18) D7S820 3′gaattataac gattccacat ttatcctcat tgac (SEQ ID NO: 19) D13S317 5′ttgctggaca tggtatcaca gaagtctggg atg (SEQ ID NO: 20) D13S317 3′ccataggcag cccaaaaaga cagacagaaa ga (SEQ ID NO: 21) D16S539 5′aaacaaaggc agatcccaag ctcttcctct tcc (SEQ ID NO: 22) D16S539 5′ataccattta cgtttgtgtg tgcatctgta agc (SEQ ID NO: 23) D18S51 5′ggtggacatg ttggcttctc tctgttctta ac (SEQ ID NO: 24) D18S51 3′ggtggcacgt gcctgtagtc tcagctactt gc (SEQ ID NO: 25) THO1 5′tacacagggc ttccggtgca ggtcacaggg a (SEQ ID NO: 26) THO1 3′ccttcccagg ctctagcagc agctcatggt gg (SEQ ID NO: 27) TPDX 5′actggcacag aacaggcact tagggaaccc (SEQ ID NO: 28) TPDX 3′ggaggaactg ggaaccacac aggttaatta (SEQ ID NO: 29)Human loci ApoB, D18S51 and Sry (primer size experiment, SI): APOB500atggtaaatt ctggtgtgga aaacctggat gg (SEQ ID NO: 30) APO500-28taaattctgg tgtggaaaac ctggatgg (SEQ ID NO: 31) APO500-25attctggtgt ggaaaacctg gatgg (SEQ ID NO: 32) APOB300REVctatccaaga ttgggctaaa cgtatgaaag ca (SEQ ID NO: 33) APOB300REV-28ccaagattgg gctaaacgta tgaaagca (SEQ ID NO: 34) APOB300REV-25agattgggct aaacgtatga aagca (SEQ ID NO: 35) D18S515-28gacatgttgg cttctctctg ttcttaac (SEQ ID NO: 36) D18S515′-25atgttggctt ctctctgttc ttaac (SEQ ID NO: 37) D18S513′-28gcacgtgcct gtagtctcag ctacttgc (SEQ ID NO: 38) D18S513′-25cgtgcctgta gtctcagcta cttgc (SEQ ID NO: 39) SRY3aaagctgtaa ctctaagtat cagtgtgaaa c (SEQ ID NO: 40) SRY3-28gctgtaactc taagtatcag tgtgaaac (SEQ ID NO: 41) SRY3-25gtaactctaa gtatcagtgt gaaac (SEQ ID NO: 42) SRY4gttgtccagt tgcacttcgc tgcagagtac c (SEQ ID NO: 43) SRY4-28gtccagttgc acttcgctgc agagtacc (SEQ ID NO: 44) SRY4-25cagttgcact tcgctgcaga gtacc (SEQ ID NO: 45) DNAs used in this disclosureBsA1 ttgggcactt ggatatgatg gaactggcac (SEQ ID NO: 46) BsA1-36ttgggcactt ggatatgatg gaactggcac ggttgt (SEQ ID NO: 47) BsA1-410ttgggcactt ggatatgatg gaactggcac ggttgttgcg (SEQ ID NO: 48) BsA1-45ttgggcactt ggatatgatg gaactggcac ggttgttgcg tccat (SEQ ID NO: 49) BsB3ccatcttcag agaacgcttt aacagcaatc c (SEQ ID NO: 50) BsB3-36cgccatcttc agagaacgct ttaacagcaa tccatt (SEQ ID NO: 51) BsB3-40cgccatcttc agagaacgct ttaacagcaa tccattttgc (SEQ ID NO: 52) BsB3-45cgccatcttc agagaacgct ttaacagcaa tccattttgc gccag (SEQ ID NO: 53) ApoB4cagtgtatct ggaaagccta caggacacca aaa (SEQ ID NO: 54) ApoB4-40cagtgtatct ggaaagccta caggacacca aaataacctt (SEQ ID NO: 55) ApoB4-45cagtgtatct ggaaagccta caggacacca aaataacctt aatca (SEQ ID NO: 56) Apo300tgctttcata cgtttagccc aatcttggat ag (SEQ ID NO: 57) Apo300-40tgctttcata cgtttagccc aatcttggat agaatattgc (SEQ ID NO: 58) Apo300-45tgctttcata cgtttagccc aatcttggat agaatattgc tctgc (SEQ ID NO: 59) SRY8ccagctgtgc aagagaatat tcccgctctc cg (SEQ ID NO: 60) SRY9cctgttgtcc agttgcactt cgctgcagag t (SEQ ID NO: 61) J1acggcattaa caaacgaact gattcatctg cttgg (SEQ ID NO: 62) K2ccttaatttc tccgagaact tcatattcaa gcgtc (SEQ ID NO: 63) NfoIprobe5′-catgattgga tgaataagct gcagc-[dTfluorescein]-g-[tetrahydrofuranyl]-t-[dT-DDQ1]-aaaggaaact ta-dRbiotin-3′ (SEQ ID NO: 64) ORFX45-bccaagaattg aaccaacgca tgacccaagg gcaaagcgac tttgt (SEQ ID NO: 65)ORFXprobe25′-(FAM)-CCACATCAAATGATGCGGGTTGTGTTAAT-[d-SPACER]-GAACAAGTGTACAGAG-3′ddC (block) (SEQ ID NO: 66) SATamira15′-tgttaattga acaagtgtac agagcatt-[dT-tamra]a(THF)ga(BHQ1)tatgcgtgga g-Biotin-3′ (SEQ ID NO: 67). SATamira25′-tgttaattga gcaagtgtat agagcatt(dTtamra])a(THF)ga(BHQ2)tatgcgtgga g-Biotin-3′ (SEQ ID NO: 68) BSFlc5′-catgattgga tgaataagct gcagc (F)g(H)t(q3) aaaggaaact ta-Biotin-3′(SEQ ID NO: 69)Sequence of MSSA and MRSA Alleles and Primers Used Here:

Primer target sites are bold/underlined, probe binding site is inbold/italic.

MRSA/MSSA primers (S. aureus experiment): (SEQ ID NO: 70) SCCI/IIctcaaagcta gaactttgct tcactataag tattc (SEQ ID NO: 71) SCCIIIccaatatttc atatatgtaa ttcctccaca tctca (SEQ ID NO: 72) ORFXcccaagggca aagcgacttt gtattcgtca ttggcggatc aaacg (SEQ ID NO: 73) MSSAccaatttgat agggcctaat ttcaactgtt agcta (SEQ ID NO: 74) sccII-35-2-bio5′-bio-ctatgtcaaa aatcatgaac ctcattactt atgat MSSA DNA sequence:(SEQ ID NO: 75) ttttagatat aaa ccaattt gatagggcct aatttcaact gttagcta ctacttaagtta tatgcgcaat tatcgtgata tatcttatat attgaatgaacgtggattta atgtccacca tttaacaccc tccaaattat tatctcctcatacagaattt tttagtttta cttatgatac gcct

 

 

 

c aacccgcatc atttgatgtgggaatgtcat tttgctgaat gatagtgcgt agttactgcg ttgtaagacg tccttgtgca ggccgtttga tccgccaatg acgaatacaa agtcgctttg cccttggg tc atgcgMRSAI DNA sequence: (SEQ ID NO: 76) tttagttgcagaaagaattttctcaaagctagaactttgcttcactataagtattc agtataaagaatatttcgctattatttacttgaaatgaaagactgcggaggctaactatgtcaaaaatcatgaacctcattacttatgataagctt

caacccgcatcatttgatgtgggaatgtcattttgctgaatgatagtgcgtagttactgcgttgtaagacgtccttgtgcaggc cgtttgatccgccaatgacgaatacaaagtcgctttgcccttggg tcatgcg MRSAII DNA sequence:(SEQ ID NO: 77) tttagttgcagaaagaattttctcaaagctagaactttgcttcactataagtattc agtataaagaatatttcgctattatttacttgaaatgaaagactgcggaggctaactatgtcaaaaatcatgaacctcattacttatgataagcttcttaaaaacataacagcaattcacataaacctcatatgttctgatacattcaaaatccctttatgaagcggctgaaaaaaccgcatcatttatgatatgctt

caacccgcatcatttgatgtgggaatgtcattttgctgaatgatagtgcgtagttactgcgttgtaagacgtccttgtgcaggc cgtttgatccgccaatgacgaatacaaagtcgctttgcccttggg tcatgcg MRSAIII DNA sequence: (SEQ ID NO: 78) aaggtataatccaatatttcatatatgtaattcctccacatctca ttaaatttttaaattatacacaacctaatttttagttttatttatgatacgctt

caacccgcatcatttgatgtgggaatgtcattttgctgaatgatagtgcgtagttactgcgttgtaagacgtccttgtgcaggc cgtttgatccgccaatgacgaatacaaagtcgctttgcccttggg tcatgcg

Example 2 Kinetics of an RPA Reaction

A schematic of the RPA process is shown in FIG. 12A. Recombinase/primerfilaments scan template DNA for homologous sequences (red/blue).Following strand exchange the displaced strand is bound by gp32 (green),primers are extended by Bsu polymerase (blue). Repeatedbinding/extension events of opposing primers result in exponential DNAamplification.

The kinetics of recombinase/primer filament formation is shown in FIG.12B. In the presence of ATP uvsX (grey) binds cooperatively tooligonucleotides (red, top). Upon ATP hydrolysis the nucleoproteincomplex disassembles (left) and uvsX can be replaced by gp32 (green,right). The presence of uvsY and Carbowax20M shifts the equilibrium infavor of recombinase loading.

The result of a typical RPA reaction is shown in FIG. 12C which is aPAGE of RPA reactions using primers for STR markers. Genomic DNA fromtwo individuals (½, father/son) served as template. Occasionally(D7S820, D16S539), low-level amounts of dimeric forms of full-lengthproduct can be observed (asterisks).

The ability to monitor RPA reaction in real time is shown in FIG. 12D.In FIG. 12D, a real-time RPA using primers for the B. subtilis SpoBlocus was monitored by monitoring the fluorescence of a reaction.Fluorescence upon intercalation of SybrGreenI into nascent product isdetected. B. subtilis DNA at 5×10⁵ (black), 5×10⁴ (red), 5×10³ (yellow),500 (green) or 50 copies (purple) or water (blue) served as template.The onset of amplification depends linearly on the logarithm of thestarting template copy number (see inset; time (midpoint of growthcurve) versus log [template concentration]).

Example 3 Detection of RPA Amplicons Using Lateral Flow Strips

We devised a method of using lateral-flow-strip technology for thedetection of RPA amplicon. This method uses specific antibodies toimmobilize and detect complexes containing two antigenic labels (FIG.13A). Briefly, a target nucleic acid is amplified using two differentoligonucleotide primers, wherein each primer comprises a different labelor antigen. Thus, all generated amplicons would be linked to two labelsor antigens (i.e., a double labeled amplicon).

To detect the presences of the double labeled amplicons, samplessuspected of containing the amplicons a pad soaked in visible (gold)particles coupled to an antibody recognizing one of the two labels (inthis case, the label is an antigen) (FIG. 13C). The complexes thentravel in a buffer stream through the membrane and an additional,immobilized antibody captures the second antigen (Id.). If the antigensare conjoined in a DNA duplex, a colored line appears at a definedlocation on the strip. In a variation of our probe detection system weproduced such dual antigen complexes by coupling Biotin- and FAM-bearingoligonucleotides in RPA amplicons (FIG. 3B). The 5′-biotinylated primerand its opposing counterpart ensure the efficient amplification of atarget for probe binding. The probe, including a 5′-FAM label, aninternal THF and a 3′-blocking group, is incised by Nfo upon binding,creating a 3′OH substrate for elongation by Bsu. The extension of theprobe remnant stabilizes its interaction with the Biotin-labeled strandand produces an amplicon that contains both, Biotin and FAM. TheTHF/3′block prevents the production of Biotin/FAM containing primerartifacts, as processing of bona fide duplexes by Nfo adds a criticalproofreading step. After application of the sample to thelateral-flow-strip Biotin/FAM-amplicons will create a visible signal onthe FAM detection line, while RPA reactions that fail to generate aconjoined complex will not. We used a multiplex approach similar to theone employed in FIG. 10E to detect 10 copies of each of the three MRSAalleles and distinguish them from MSSA (FIG. 3C).

A number of research and clinical applications could benefit fromemploying RPA and the various detection methods disclosed herein. Forexample, RPA offers a significant breakthrough for the development ofnon-laboratory devices. When integrated with handheld instruments orentirely equipment free DNA detection systems, RPA will enable aneasy-to-use testing system for a variety of pathogens as well as fieldkits for other applications.

Materials and Methods

Proteins and DNA

Coding sequences for uvsx, uvsy, gp32, Bsu and Nfo were amplified fromgenomic DNA (DSMZ, Germany), fused to hexahistidine-tags (N-terminal foruvsY, Bsu and Nfo, C-terminal for uvsX and gp32) and cloned intosuitable expression vectors. Overexpression and purification was done bystandard protocols using Nickel-NTA resin (Qiagen).

Human DNA was purified from blood (Wizard-Genomic-purification-kit,Promega), B. subtilis DNA was from ATCC (USA), S. aureus DNAs were agift from Jodi Lindsay. S. aureus alleles were EMRSA-3 (SCCmec type I;MRSAI), EMRSA-16 (MRSAII), EMRSA-1 (MRSAIII) and wild-type MSSA (12).See supplementary information for sequences.

RPA Conditions

Reactions were performed at 37° C. for 60 min or as indicated. Standardconditions were 50 mM Tris (pH 8.4), 80 mM Potassium-acetate, 10 mMMagnesium-acetate, 2 mM DTT, 5% Carbowax20M, 200 μM dNTPs, 3 mM ATP, 20mM Phosphocreatine, 100 ng/μl Creatine-kinase, 20 ng/μl Bsu. Incontrast, MRSA amplifications were done at 50 mM Tris (pH 7.9), 100 mMPotassium-acetate, 14 mM Magnesium-acetate; in the multiplex experimentCarbowax20M was at 5.5%. Concentrations of gp32/uxsX/uvsY (in ng/ul)were 600/200/60 (STR experiment), 600/120/30 (B. subtilis experiment) or900/120/30 (MRSA experiments). Primers were employed at 300 nM each,except in MRSA amplification, where 500 nM sccIII, 100 nM orfX (MRSAIIIexperiment) or 265 nM sccI/II, 265 nM sccIII, 70 nM orfX (multiplexexperiment) or 240 nM BiosccI/II, 240 nM Bio-sccIII, 120 nM orfX(lateral-flow-strip experiment) have been used. Reaction volumes were 20μl, except for the STR experiment (40 μl) and the B. subtilis experiment(50 μl).

Real-Time Monitoring

Real-time RPA was performed in a plate-reader (BioTek Flx-800) in thepresence of SybrGreenI (1:50000, Molecular Probes) orfluorophore/quencher probes (Eurogentec). Three probes were employed:

SATamra1 5′-tgttaattgaacaagtgtacagagcatt(T)a(H)ga(q1)tatgcgtggag-Biotin-3′ SATamra25′-tgttaattgagcaagtgtatagagcatt(T)a(H)ga(q2)tatgcg tggag-Biotin-3′ BSF1c5′-catgattggatgaataagctgcagc(F)g(H)t(q3)aaaggaaact ta-Biotin-3′

Here (T) is dT-TAMRA, (F) is dT-Fluorescein, (H) is THF, (q1) isdT-BHQ1, (q2) is dT-BHQ2, (q3) is dT-DDQ1. Probes were employed at 60 nMSATamra1 (MRSAIII experiment) or at 45 nM SATamra1, 45 nM SATamra2, 60nM BSFlc (multiplex experiment). Nfo was used at 200ng/ul.Excitation/detection was at 485/525 nm (SybrGreenI, BSFlc) or 530/575 nm(SATamra½). Measurements were taken every 30 sec or 45 sec (multiplexexperiment). Fluorescence probe data were normalised against watercontrol and pre-amplification baseline adjusted. The logarithm of theread-out was plotted against reaction time.

Lateral-flow-strip detection

For lateral-flow-strip experiments two probes were used at 75 nM each:

For lateral-flow-strip experiments two probes were used at 75nM each: Lfs1 5′FAM-ccacatcaaatgatgcgggttgtgttaat(H)gaacaagtgtac agag-ddC-3′ Lfs25′FAM-ccacatcaaatgatgcgggttgtgttaat(H)gagcaagtgtat agag-ddC-3′

5′-biotinylated forms of sccI/II and sccIII were utilised as primers.For each reaction (20 ul) 1 ul was diluted with 5 ul running buffer(PBS/3% Tween) and applied directly to HybriDetect-strips (Milenia)according to manufacturer instructions.

The result of the lateral flow strip detection is shown in FIG. 13C.Reactions contained (left to right) 10 copies MRSAIII, 10 copies MRSAII,10 copies MRSAI or 10000 copies MSSA (negative control) as template.Positive signals are generated in the first 3 reactions (arrowhead).

Example 4 Analysis of Optimal Conditions for RPA

RPA Conditions

RPA relies on the establishment of a reaction environment that supportthe formation of recombinase-oligonucleotide complexes. Since theprocess is also ATPdependent (Formosa et al., 1986), it requires anenergy regeneration system for sustained activity. In this experiment,we titrated key components of the RPA reaction mixture in order todetermine their influence on amplification performance. The results areshown in FIG. 14 . FIG. 14 shows polyacrylamide gel electrophoresis ofRPA reactions using primers for the human Sry locus. Reactions wereperformed at 37° C. for 120 min and contained the primers sry3 and sry4at 300 nM, 50 mM Tris (pH 8.4), 80 mM Potassium-acetate, 10 mMMagnesium-acetate, 2 mM DTT, 3 mM ATP, 200 μM dNTPs, 20 mMPhosphocreatine, 100 ng/μl Creatine-kinase, 5% Carbowax20M, 600 ng/μlgp32, 200 ng/μl uvsX, 60 ng/μl uvsY and 20 ng/μl Bsu, except when agiven component was that under investigation. Optimal quantities of(FIG. 14 A) gp32, (FIG. 14 B) uvsY, (FIG. 14 C) uvsX, (FIG. 14 D)Carbowax20M, (FIG. 14 E) ATP and (FIG. 14 F) Bsu for effectiveamplification of this particular target were determined. (G) ADP-®-S and(H) ATP-©-S inhibit the reactions. 1500 copies/pi of the humanY-chromosomal DNA served as template in 30 ul reactions (per sample theequivalent of 10 ul reaction volume was loaded on the gel).

RPA proved to work robustly over a relatively wide range of reagentconcentrations. We found, however, that optimal reaction conditionsvaried between different primer pairs and therefore had to be definedindividually.

Primer Requirements

We used RPA to amplify of a wide range of targets. While the design ofprimers revealed no limitations on sequence composition itself, certainparameters have to be met for an oligonucleotide to be suitable for RPA.We investigated these parameters in the experiments shown in FIG. 15 .FIG. 15 shows agarose gel electrophoresis of RPA reactions using primersfor the human Apolipoprotein B locus. Primer ApoB4 was combined withopposing primers capable of generating products of the indicated sizes.Reactions were performed at 37° C. for 120 min and conditions used were50 mM Tris (pH 8.4), 80 mM Potassiumacetate, 10 mM Magnesium-acetate, 2mM DTT, 3 mM ATP, 200 μM dNTPs, 20 mM Phosphocreatine, 100 ng/μlCreatine-kinase, 5% Carbowax20M, 600 ng/μl gp32, 125 ng/μl uvsX, 25ng/μl uvsY, and 20 ng/μl Bsu. 450 copies of human DNA were used astemplate in 30 μl reactions (per sample the equivalent of 10 ul reactionvolume was loaded on the gel). Note that some hairpin-mediated productduplication occurred, converting some of the 300 bp amplicon to 2× and3× unit length (*). RPA failed to produce amplicons of 1500 bp or more.This experiment shows that amplicon size under the conditions employedis limited to approximately 1 kb.

Shown is polyacrylamide gel electrophoresis of RPA reactions usingprimers for the three independent loci in human genomic DNA(Apolipoprotein B, STR D18551, Sry). Primers were 25, 28, or >31 bases,as indicated. Reactions were performed at 37° C. for 120 min. Conditionsused were 50 mM Tris/Cl pH 8.4, 80 mM Potassium acetate, 10 mMMagnesium-acetate, 2 mM DTT, 3 mM ATP, 200 μM dNTPs, 20 mMPhosphocreatine, 100 ng/μl Creatine kinase, 5% Carbowax20M, 600 ng/μlgp32, 200 ng/μl uvsX and 60 ng/μl uvsY, and 20 ng/μl Bsu polymerase.3000 copies of target served as template in 30 ul reactions (per samplethe equivalent of 10 ul reaction volume was loaded on the gel). Thefinding that a primer length of >28 bases is required to support RPA isin good agreement with reports that investigated the ATP hydrolysisactivity of uvsX-oligonucleotide filaments at different oligonucleotidesizes (See, Huletsky et al., 2004).

The minimum length of a primer proved to be about 30 nucleotides (FIG.16 ). We observed variability in the performance of oligonucleotidesthat differ in sequences but are similar in length and position relativeto their counterpart. The rules governing the influence of nucleotidesequence on the quality of a particular RPA primer are currently underinvestigation.

Control DNA

The wild-type S. aureus DNA (MSSA) (See, Enright et al., 2002; Huletskyet al., 2004) serving as a negative control in the experiment shown in2C does act as a template for RPA when combined with the primer pairorfX/mssa (FIG. 16 ).

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The invention claimed is:
 1. A method comprising: providing a liquidsample comprising a target nucleic acid; combining the liquid samplewith a recombinase polymerase amplification (RPA) reagent composition toprovide a reaction mixture, wherein the RPA reagent compositioncomprises: a recombinase; a polymerase; a first nucleic acid primer; asecond nucleic acid primer; a third extension-blocked primer comprisingan elongation blocking group at a 3′ end, an internal tetrahydrofuran(THF) residue, and an internal residue non-complementary to the targetnucleic acid; and a nuclease; performing RPA on the reaction mixture toproduce an amplicon indicative of the presence of the target nucleicacid; and detecting the presence of the amplicon using a lateral flowstrip.
 2. The method of claim 1, wherein the lateral flow strip isconfigured to form a visible signal at a specific position on thelateral flow strip indicating the presence of the target nucleic acid.3. The method of claim 1, wherein the lateral flow strip comprises asample pad, a detection line, and a control line, wherein said detectionline is positioned upstream of a control line.
 4. The method of claim 3,further comprising transferring all or part of the reaction mixture tothe sample pad; flowing the reaction mixture to the detection line; andcapturing an amplicon indicating the presence of the target nucleic acidat the detection line.
 5. The method of claim 1, wherein said firstnucleic acid primer is labeled with a first detectable label selectedfrom the group consisting of enzymes, enzyme substrates, coenzymes,enzyme inhibitors, fluorescent markers, chromophores, luminescentmarkers, radioisotopes, and a member of a binding pair; and/or saidsecond nucleic acid primer is labeled with a second detectable labelselected from the group consisting of enzymes, enzyme substrates,coenzymes, enzyme inhibitors, fluorescent markers, chromophores,luminescent markers, radioisotopes, and a member of a binding pair. 6.The method of claim 5, wherein an amplicon comprises the firstdetectable label and/or the second detectable label(s).
 7. The method ofclaim 6, wherein said amplicon comprising the first detectable labeland/or the second detectable label(s) produces a visible signal on thelateral flow strip.
 8. The method of claim 5, wherein said first nucleicacid primer and/or said second nucleic acid primer is/are labeled with amember of a binding pair that is a biotin, an avidin, a streptavidin, anantigen, an antibody, a ligand, a receptor, an analog or a mutant of abiotin, an analog or a mutant of an avidin, an analog or a mutant of astreptavidin, an analog or a mutant of an antigen, an analog or a mutantof an antibody, an analog or a mutant of a ligand, or an analog or amutant of a receptor.
 9. The method of claim 7, wherein the visiblesignal is a fluorescent signal.
 10. The method of claim 1, whereinperforming RPA comprises maintaining said reaction mixture at atemperature of at least 30° C., at least 35° C., or at least 40° C. 11.The method of claim 1, wherein performing RPA comprises maintaining saidreaction mixture at a temperature less than 45° C., less than 40° C.,less than 35° C., or less than 30° C.
 12. The method of claim 1, whereinperforming RPA comprises maintaining said reaction mixture at atemperature for a time period of approximately 5 minutes, approximately15 minutes, approximately 30 minutes, or approximately 15-60 minutes.13. The method of claim 5 wherein said first nucleic acid primer andsaid second nucleic acid primer are labeled with different detectablelabels.
 14. The method of claim 1 wherein said lateral flow stripcomprises an antibody.
 15. The method of claim 1 wherein said liquidsample comprises blood.
 16. The method of claim 1 wherein said liquidsample comprises less than 100 copies of said target nucleic acid.